The best game EVER,huh...theoretically speaking it is :
Action-Adventure :
The legend of Zelda : Ocarina of Time[Hands down,no comment]
Action :
...that's a toughie...
Extreme Combat/Stylish Action :
Bayonetta,but I believe that Sengoku Basara 3 will be on par with it.
Mecha Action Game :
The Another Century Episode Series...all three of them.No question about it.
Stealh game :
Tenchu 3
Hack'n'Slash game :
That's a toughie...maybe DMC1,since it really was between the line of Stylish action and H'n'S...
Strategy RPG :
Thus far Super Robot Wars F Final.
RPG :
Fina Fantasy VII[Hands Down]
Action RPG :
Another Toughie...though I believe it would be Ys 7...if it actually counts as one.[Yes it's that tough to categorize the YS series].
Real Time Strategy :
Dunno.
First Person Shooting Game :
The Metroid Trilogy for sure.
Shoot'em Up/Stylish gunfight action game[Yup it is a different category from the Stylish action games] :
Gungrave Overdose[Hands down]
2D Beat'em Up :
Many will rage I know but...Ikkitousen Xross Impact.
3D Beat'em Up :
Must be Hokuto Musou[Fist of the North's Star : Ken's Rage in the west]
2D Fighting :
Guilty Gear XX Accent Core +[Hands down]
2.5D Fighting game :
PS : These are mostly anime games so take this with a grain of salt...
...I say Fate/Unlimited Codes...if not it is Tatsunoko Vs Capcom.
3D fighting games :
Ok,that is extremely tough...I seriously don't know...
Visual Novel/Adventure Game :
Fate/Stay Night[Hands Down]
PS : Demonbane...*sniff*...next time...next time...
2D Platform Game :
A Metroidvania game...seriouly that's like 90% of this game's category...I dunno pick what you like.
3D Platforming game :
Quite possibly A crash Bandicoot one...Crash 2?Or maybe Spiderman for the PSX.
Dramatic ACT games[A visual Novel sub category] :
Kishin Hishou Demonbane.[Hell yes.]
Anime Game[Not many games are in this category surprisingly!It is for anime games with their very own game engine] :
Rurouni Kenshin : Enjou Kyoto Rinne.
I can't think of any other games in other gaming categories...
Now out of all of them...Ocarina of Time and FFVII,still argue about the number 1 spot in the gaming pantheon.
PS : This is theoreticall and mostly neutral.It does not reflect my tastes.