In my experience with hobby games, a 'universal system' is not an ideal good. After 20 years of 'DBx', for instance, I think the only reason I can stomach another variation on the theme is because that has remained a change of pace for me. Risus can be handy shorthand for scenario ideas, but I have no interest in actually playing it. [There's a common thread running through Chaosium's games, but rules differences are also part of their various flavors.]
There have, however, been quite a few splendid all-in-one rpgs, including the early offerings such as D&D and Traveller that were in one box of several booklets.
FGU published very comprehensive works. They could be a bit imposing, though, and often were not aimed at the novice or casual player.
Brevity seems a critical distinction. A rule-book that stops bullets is no demonstration of great game design. Neither is tiny type, as in the first editions of Chivalry & Sorcery and Metamorphosis Alpha.
Among Chaosium's many insanely great releases, I will cast my vote for Worlds of Wonder. That kept the 'Basic' really first in BRP, and did a pretty good treatment of Superworld in less than 20 pages (with similarly concise workups of Magic World and Future World).
CoC is simply nonpareil, of course.
The trouble with Chaosium generally is that they put out such spiffy supplements that not 'needing' more than the core is probably not the issue. Then again, later editions of the basic set have tended to add in material.
Every edition of King Arthur Pendragon is beautiful, but I find it hard not to want at least the overall campaign supplement (e.g., The Boy King) in addition. Even harder would be to pick up Runequest and pass up Cults of Prax.
GDW's classic Traveller was similar in that regard, with fine additions also from the likes of FaSA and Digest Group. However, any of the versions of the basic set (including The Traveller Book) will provide for a galaxy of adventure. Part of that -- as with D&D -- depends on grooving on the literary sources of inspiration. Mongoose takes another approach with supplements specifically for, e.g., Strontium Dog or Babylon 5.
TSR's Empire of the Petal Throne introduced a world that has been documented at least as extensively as Middle Earth. The original game, I think, conveys a lot of flavor without bogging down in encyclopedic minutia. At the same time, it anticipated Holmes Basic in offering a clear explanation of how to play.
Gangbusters gave us the era of Prohibition, perhaps not from as many perspectives as the designers had hoped to include -- but still a great 'sandbox campaign' basis. The adventure modules maintain the high standard of quality, but it's not as if there's likely to be a shortage of plots afoot once players start playing.
Some supplements for FGU's Flashing Blades might be more intriguing, but the basic game is another rich milieu to explore.
Marvel Super Heroes is a blast -- but partly because of all the write-ups of people, places and things from the Marvel Universe of that time. Love those supplements too much!