Best game on PS3 in regards to gameplay, story line, sound quality and interactive game experience is MGS 4.
MGS 4 is the only MGS game I ever played and it is a great game. No need to play the previous MGS games while they would look dated in comparison to MGS 4. MGS and MGS 3 I heard are great older MGS games but MGS 2 I have heard had mix reviews from gamers on that game ranging from good to bad.
Runner-ups to next best exclusive games on PS3 are Uncharted(next gen tomb Raider style game), Little Big Planet(innovative platform game) and Killzone 2(best graphics, great online but lacking on story line in comparison to Killzone on PS2). Infamous (brilliant game where you are a super hero/villain - good or evil your actions have consequences in your world) Gran turismo 5:Prologue(expensive demo/preview to Gran Turismo 5 - best racing series on PS3). Warhawk - Its a good game for someone looking for a challenge on PS3 - online only and difficult game is the drawback to it.
PS3 exclusive games to watch are: God of War 3(continuation of the excellent top of the pile Hack n' Slash series God of War), UnCharted 2(next chapter in the new PS3 action game series UnCharted) Gran turismo 5 - definitive racing simulation game exclusive to Play staion console, MAG - 256 online shooter game(could be a master piece or a disaster - hit or miss), Final Fantasy Versus XIII (action JRPG exclusive for the PS3 and slightly better than Final Fantasy XIII according to top game analysts), Final Fantasy XIV - MMO JRPG exclusive for PS3 - you will either love it or hate it just like FFXI another MMO JRPG.
Lots of excellent multi-platform games exist on the PS3 or are soon to be released on PS3. Multi-platform games are equal on both PS3 and 360. Anyone who says differently is lying. Multi-platform standouts on PS3: GTA IV, Resident Evil 5, Fallout 3, The Orange Box, Devil May Cry 4, Fifa/PES series, NBA/NFL series, Burnout series, Call of Duty series, Assassins Creed series, Biosock series, etc