The game play, mechanics and everything about the game is top notch. When i can replay a game 25+ times and not really get bored that's awesome.
My favorite thing though is how they all fit perfectly together. The playable characters from the 1st game are NPCs in the 2nd and the playable characters from the 2nd game are NPCs in Tales. The whole thing was done in a way that makes me believe Gearbox planned everything out from the start and just cut it into installments.
If you've never played stop whatever non important thing your doing now and go play them all. GOTY if possible.