that's a tough one
hmmm... lets see...
I'm only familiar with the prime series and zero mission
I liked all of them
Prime was just epic being the first one and all that, the feeling you get is the basis for the series
Echoes was an worthy succesor with more classic upgrades and awesome stroyline
Corruption was a bit to easy for my taste, but fortunately they invented hypermode ^^
very epic game.
Zero mission, also very epic
I think it would be...
Corruption!
The WII added a lot to the gameplay
and the overall combination of worlds, storyline, cutscenes etc was well balanced.
It was a bit short/smaller then the others, but nonetheless fun.