The M&M installment with the best storyline.

The game world is the same in all games of the M&M series.

No, the world's history is the same in all parts (or rather, it's a common thread), but the world itself is different, as it was created differently by the developers. If we take a hypothetical M&M6.5 game (:D) that consists of playing through one map with one quest and compare it to M&M7, then although the universe of both games will be the same, the game world will be significantly different.
So, in M&M6, it is much more detailed, in my opinion.