I have many different RPGs in my collection. There are many I like. But I have only a few favourites. How I determine a favourite is simple, would I actually run a game with it. My favourites are subdivided into several categories. Generic is self explanatory. Genre games are one that deal with a type of fictional universe but without having a specific setting detailed. For example Dungeons and Dragons is a genre game. And finally setting-specific games. Games built around a detailed background, ie Shadowrun. Here is the short list of my favourite generic games.
F U D G E , is not so much a complete set of rules as a toolkit. It is free, although deluxe printed versions are available for purchase. It has a funky dice system and its task resolution system can be translated into real words for ease of play with non-role players. Its a good transition from traditional RPGs but it requires much prep work for the GM due to its toolkit nature.
Story Engine, has simple mechanics and a cool health system. Very cinematic with a flexible descriptive character creation system. Works better with more experienced players as the game requires greater than average player input. It has an simpler intro version called Story Bones that takes up only six pages. This was my top favourite until recently and remains my solid second choice.
Risis, also only takes up six pages and despite not taking itself seriously is a well designed game. It requires the most player input from both the clever character creation system to the cinematic and dramatic style of play.It does sacrifice alot on detail to achieve ease of play and some traditional gamers might find it unsatisfying. The six page rules are free and there is a 50 page "companion" for sale by the game's creator.
My interest in generic games goes back to the late 80's when I first got into GURPS. It didn't make this list because over the years I began to see the game as clunky and unwieldy. Especially with the newer 4th edtion.