Best game fanbase?

X series from Egosoft I'd say.
Of course, it is a totally biased point of view :p

can anything compare to Sonic & Mario franchise?

Lord of the Rings Online.

unreal and hl2 have community mod almost infinite ^^

Neverwinter Nights.

• The modding community is still active after eleven years.
• There are extensive wiki's dedicated to various aspects of the game.
• The vast majority of people in the community are still friendly, helpful, and enthusiastic about the game. (Trolls are rare.)
• The number and quality of persistent worlds (basically free MMO's) is notable.
• The Diamond version of the game is still selling, which means people will still be involved with the game for a while to come.

league of legends

Best in terms of what? Size? Or quality of membership?

For size I'm going to say Mario.

Quality of membership... Hard to say. Every barrel has some rot in the bottom. I'm going to say The Elder Scrolls, because of the mods, and the work to bring the older games into the engine of the newest one.