Not to be afraid to wade into a possible flame war but....
There are no, IMO anyway, Best Games
Every game serves a different purpose and while they are all Roleplaying games they cater to different needs.
Now on a purpose built basis
GURPS is better for simulationist gaming and has better sourcebooks. Sorry Avalanche ;
BESM is better for rules lite, heroic gaming
RISUS is better for almost rule-less or almost silly gaming
FUDGE is a better tool kit
UNISYSTEM is better for Urban Fantasy. OK I am biased about that as I playtest for Eden but damn thats a good system and well written.
OTOH
D20 is better for actually gettting players
D20 is better for "one man army" characters
D20 is better than Palladium. although Palldium was better by far than 1e and slightly better than 2e. However Palladium hasn't evolved and D&D/D20 has. Also D20 is balanced unlike Palladium
D20 is about on par or slightly better than Rolemaster 3 or 4 Rolemaster has evolved and is still a great game but personal blind spot aside D20 is easy to set up and design characters for. Also with a few minor patches feels much the same. D20 is also widely played and faster to play as well.
D20 especially D20 SAS is better than Hero for Supers gaming. Ths will be controversial to some but D20 is so much easy to use in actual play than Hero it just isn't funny. Oh and for the record I have owned Champions 1,2,3 (when they were seperate books) Fantasy hero (very good) Beastiary, Robot Hero, Danger International and more. It is a good system but D20 beats it hands down
D20 is Better than BRP becuase of the variety. BRP (Call of Cuthulu, Elric, Runequest) has never had a basic set of genere games. While BRP is quite good, D20 with slight mods (Grim N Gritty plus class tweaks) has much the same flavor. To tell you the truth I don't like making up a lot of rules for a game and for example if I wanted to use BRP for generic Fantasy I would have a lot of work to do, not fun. Also BRP is not easily adjusted for letality. It was brutal enough that many players called it Limb Quest --- Very nasty
If I had to do the desert island thing and had to pick one game book (to go with my Tyvek copy of How to Survive, Thrive and Escape a Desert Island 3rd ed revised) it would be the hypothetical BESM expanded edition because it can do anything however D&D3e is always on my top systems list
I will make a confession to you all, until recently I hated D&D and in fact was on the bashers list. Oh I played 2e (won't play 1e) and had some good sessions but I thought D&D was highly lame.
3e cured me of that feeling, I like the rules and enjoy playing. Now I prefer Kens Grim N Gritty rules but I enjoy D&D core for the most part.
Hats off to the writers, all of 'em for the conversion. They took a game I hated, fixed it to their satisfaction and by golly even a detractor now likes it. This happened without alienating most of the play base.
No small task