If I were to advise anyone on the merits or best starting point of D100 TTRPG’s, I would have to think on it. Hard.
There are a lot of games to pick from, periods covered, several strong systemic threads and of course personal preferences.
Although most D100 games have common roots, it is probably best at the start to commit to just one of these threads for consistency and ease of adapting. It is somewhat true to say, learn one and you have started on them all, but also, some do have very coherent pathways to other games, making the job more of learn one and you can instantly play a few rhater than herding several similar systems together, but find the differences are minor, but many.
The Basic Roleplaying stream includes Call of Cthulhu, Runequest, many older games such as Stormbringer, Super World etc. It also has several generic books available, one in print currently others as pdfs, which could conceivably be the only book needed if the GM wants to build worlds or simulate stories from scratch.
CoC 7e and Rivers of London have changed the BRP mechanics a little recently, but most are easy enough to adapt and Runequest has gone “old school” (RQ2) with its latest version.
The RQ6/Mythras stream includes reasonably effortlessly the earlier Legend series (RQ3 with the RQ labels removed), RQ6, as well as many other recent releases and even better than BRP, they are all fully compatible. Destined, After the Vampire Wars, Lyonesse, M-Space, Odd Soot, several historical settings including Rome, Samurai, Vikings, Pirates, Mythic Britain, Polynesia, and more are all either Mythras, Legend or Mythras Imperative based. In other words, pretty much the same.
All different, all hauntingly similar.
Openquest is a lesser known “lite” version of RQ6, with a few good options. This is the rules lite starter range, but includes games like Clockwork and Chivalry, Pirates and Dragons and its own fantasy range. There is no generic rules system, but it can cross into BRP territory easily enough.
DWD’s Bare Bones systems are also lite and a little different to the BRP family, but broadly compatible. There are consistency issues within their own range and some off-shoots also, but basically they are lite enough to not matter.
The Warhammer family is quite diverse. WHFRP 1 and 2e are D100 variants, 4e is the latest and current offering, but quite a different beast (we don’t talk about 3e). Zweihander is a rip-off of 1e, fully polished and expanded without the thematic baggage.
These are the main ones, but there are others.
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First, if there a genre of preference, something to pin your recommendation on, it would likely sort things easily as some periods and styles are not heavily serviced.
Horror in the classic periods (1890’s, 1920’s and WW2) is easy. Call of Cthulhu 7e. I like all editions for their various feels and rules weight, 5e being the most commonly used up to lately, but 7e is current, elegant, comprehensive, very well supported and beautiful to look at. If the earlier period appeals, Cthulhu by Gaslight (5-6e) adds much, as do the excellent Stygian Fox support books and Weird War Cthulhu is suitably dark.
Horror/weird WW2 has two candidates. Both CoC 6e with World War Cthulhu for dark and realistic option and it continues with the even darker Cold War Cthulhu. This is CoC as normal, brooding, perilous. The other is Achtung Cthulhu, a pulpy option, which could also be played with 7e Pulp with little effort.
Pulp Horror is CoC 7e’s territory. BRP also has a pulpy expansion, but it is a thin offering compared to CoC 7e and it is not well supported except for Achtung Cthulhu a CoC 6e spin off. Pulp for 7e has the added benefit of being included in most CoC 7e thinking, so it is a very flexible offer.
Horror/modern. This one is tougher, but if you are after just this genre, Delta Green is the one with its companion The Conspiracy if you want a more late 90’s X-Files feel. If the period is to be only one option among others, then it is CoC 7e again, for the greater range using the Chaosium and Stygian Fox offerings. Sigils and Signs has some cred here, but is a single book offer.
Modern Fantasy/Supernatural. Most of these are to scratch a current itch of a book or TV series just binged. The Mythras based After The Vampire Wars is a Harry Dresden clone, Delta Green The Conspiracy is great for X-Files, CoC 7e (pulp?), or maybe BRP:UCE can do Supernatural and Evil is perfect for Sigil and Signs. The Laundry books have an actual D100 game as does the Rivers of London series, Seasons of the Dead for Mythras handles Falling Skies or the Walking Dead. DWD Spec Ops with Bare Bones Fantasy and/or Art of Wuxia would work as a monsters meet normals mash up.
Horror from other periods. Clockwork/Cthulhu and Chivalry are Openquest derivatives, but CoC 7e comes through again with Rome, the Dark Ages, Gaslight/Western, 1920-40’s to the far future effortlessly.
Classic Fantasy has several options. Mythras is the Audi TT, Legend to older model Audi Quattro, Runequest the classic Rolls Royce remade, Openquest the zippy Toyota and BRP’s Magic World the reliable old Jeep.
Mythras leads to several excellent games like Lyonesse and Mythic Britain, with a lot of Legend and RQ6 titles to draw from. It even does an “old school” homage to the earliest games with Classic Fantasy a DnD as D100 mash up, so overall, unless a specific setting draws you, it is the most comprehensive and polished, but a seriously deep system. It can be run as Imperative, simply reducing options, so maybe that is a vote in it’s favour.
Bronze Age is a tougher one. Mythras is pitched here as was RQ6, showing their roots as RQ evolutions. Runequest in Glorantha is the obvious one and a (the) classic of its type, but also, if you want something lighter (a bit), then try Jackals from Osprey, an Openquest derivative that deserves some attention and like Mythras is probably closer to the classic Mythic Greek take than Glorantha. Mythras owes us Mythic Greece, which may change things, but seems an eternity away.
Gunpowder Fantasy. Most of the above could do this easily enough, but Warhammer is the best supported overall. Which version? 1-2e have their charms, Zweihander copies them and cleans them up and 4e is the current big thing, but maybe too system weighty. Alternatively Clockwork and Chivalry/Cthulhu and Renaissance/Dark Streets offer a much rules lighter (and darker) feel.
Pirates, which can tie to the above, have plenty to choose from, but none are without their issues. Pirates and Dragons is a strong offering, but comes with a fantasy backstory. Pirates of Legend is excellent and Mythras compatible, BRP Blood Tides has some issues, but adds to either of the above well and with the BRP generic book cleans up well. Even WHFRP 4e is capable here.
Hard Sci-Fi. This is a simple one really, M-Space. Add some Mythras based nasties as Aliens etc and you have a solid scenario.
Pulpy Sci-Fi. DWD do Frontier Space, which stands out even from their own range as a slightly more developed game with two main books and lots of support. M-Space could also, but the BRP:UCE or even one of the Mythras based systems like Luther Arkright might suit better.
Oriental games have two very different contenders. Samurai of Legend is lethal, realistic and Japan focussed, so ideal for a Shogun or 7 Ronin style game with Mythras. The Art of Wuxia is a Chinese inspired full Kung Fu pulp game by DWD for a Monkey Magic feel.
Supers, my personal holy grail for a long time, but street level mainly is right in the wheel house of Destined for Mythras and the added complication and flaws of that game play well with the epic fights Supers games, comics and movies are known for.
The older Super World is dated for sure, but it is also the basis of the BRP:UCE super powers system, which can be meshed with Magic, Psionics, Mutations etc to make for a modern game with a small but decent back catalogue of support materiel. My copy is a printed PDF of the main book and companion and it is fine.
Western/Weird West is under the solid purview of CoC 7e Down Dark Trails, but Devils Gulch and Aces High for earlier BRP are compatible.
As you can see, there are a lot of entry points if you use the genre as the doorway.
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So, lets assume a genre specific itch does not need scratching;
The safest and most logical path for a beginner wanting to go in seriously is I think a BRP based system using either one of the excellent core games (CoC/RQ) or the new generic book.
Probably CoC 7e as it is easier to enter and makes the most gaming sense to newer players with a ton of support for GM’s (plays like a “don’t go down there” scenario generator with literally dozens of scenarios). It is also flexible on going, able to shift to lighter (Pulp) or heavier (Modern/Dark) and all through history, even into science fiction.
If you really want to go with an entry point system, the Openquest family are solid and clean, the DWD series are similar, but split into four main themes.
The cheapest probably is Legend by Mongoose. A generification of Mongoose RQ and pre-curser to RQ6, then Mythras, it is available for $1 as a PDF, not much more as an A5 book and has some excellent support books (Ice Age, Pirates, Samurai, Gladiators, Historica Rodentia, Deus Vult and Vikings) with a more high Medieval base line than other D100 fantasy games. It is not as polished as Mythras, but not as baked in to it’s ideas either.