I'm just going to post stuff here until someone joins in.
Basic RolePlaying is at the current time an embarrassment of riches. Which is good, but confusing. While the level of compatibility between the different incarnations is very high, there are differences - some subtle, some not so much.
I'll list some in an attempt to help:
Basic RolePlaying Core book (aka the BGB) - an in-depth presentation of the core d100-rules, backed up by different optional rules en masse to make it cover anything from stone age dino-riders to transalien dimension-hoppers.
Complexity 30-90%, depending on how many options are used
Magic World - building on the rules and priciples laid out in the BGB, Magic World shows how gritty, yet high, fantasy is done.
Call of Cthulhu - I don't really need to say anything about about CoC, do I? It is one of those games that everyone should at least try. Built on the options from the BGB (or vice versa, depending on how you view it).
RuneQuest - currently in its 6th edition, RQ was the original BRP. Made to be a "realistic" (and therefore very gritty) and detailed system for playing in the fantasy-world of Glorantha, it almost died at the end of the last millenium as Glorantha was styled for another game-system and RQ languished in publisher's limbo. Then it was clumsily resurrected by Mongoose, resulting in MRQ (available as a free download to peruse), fixed in MRQII and, after another change of publishers and licenses, made pretty near close to perfect as RuneQuest6.
Of note here is Legend, which is the core of Mongoose's RuneQuestII-mechanics released under a new name and OGL.
OpenQuest - a simpler, streamlined version of RQ made official. OGL and tailored for fantasy.
Renaissance is built on OpenQuest, but focuses on the the age of steam and blackpowder.
There are also some very good settings available. A few are:
Rome- Life and Death in the Republic (BRP) tells you things about the Roman Republic that you didn't think you'd ever needed to know.
The Green (BRP) is a "deep" fantasy jungle, laden with weirdness and lore.
Aces High/Devil's Gulch (BRP) - weird westerns.
Monster Island (RQ6) is a nightmare mix of the darkest fantasies of diverse pulp-fantasy writers, directed by H.Harryhausen.
Luther Arkwright (RQ6) - steampunk dimension-hopping scifi with a stiff upper lip.
The jazz-era (CoC) is the classic Call of Cthulhu-setting. And classic for a reason. Check out Masks of Nyarlathotep, Beyond the Mountains of Madness or Tatters of the King for some epic horror-campaigns.
Delta Green (CoC) The best campaign-setting ever written? Perhaps not, but it is in the top three. Mixing HPL's cosmic horror with modern conspiracies, Delta Green is Mulder&Scully w/ an ex-SpecOp sidekick vs. Cthulhu (and no, there will be no kissing at the end..). Soon to be re-released with its own BRP-derived system and updated (the 90's were 20 years ago).
Cthulhu Rising (CoC/BRP) Basicly Aliens - the Roleplaying Game, much of it unfortunately unavailable as the author has pulled all resources except for a few monographs available from Chaosium. Hopefully it will resurface.
Clockwork & Chivalry (Renaissance) Originally a setting for MRQII, it was re-released with its own house-system. An alternative english civil war, where magic-wielding alchemists are opposed by clockwork-creations while the common soldier still dies in the mud.
River of Heaven (OpenQuest) transhumanist hard scifi
Vikings of Legend (Legend) Originally a supplement for MRQII, VoL covers the ancient scandinavians in exacting detail.