Assassin's Creed was a shitty, boring game that tried new things most of which sucked. I mean WhoTF thought of the horse riding/disguise bollocks? Altair was a rubbish, uninteresting character. AssCreed 2 was a very good game that took the bits that worked (ie. the running/climbing), lost the shitty parts of the first that didn't and gave us an interesting, well realised character.
BG2:SoA/ToB over 1. Better engine, larger world and ability to level beyond 8
GTA: Vice City was much better than its predecessor and truth be told, its successors too. It nailed everything one expects from a sandbox GTA game.
Far Cry 3 is a great sandbox that improves on both of its predecessors in most ways.
Games where the opposite is true
Crysis 2/3 turned a unique, cutting edge PC shooter into a console MMS complete with handholding, checkpoints, set pieces and "staying frosty". After such a stellar title, the sequels were crushingly disappointing.
Dead Space 3 ruined and likely killed off the "franchise" entirely. There's almost nothing good about this game, except possibly...no, nothing.
Dragon Age II, while not a terrible game, was far from a great game. A rushed game with so little in common with its predecessor. Instead of a "spiritual successor" to the legendary Baldur's Gate, and the fantastic premise of the Origin stories, we got WoW gameplay mechanics, barely coherent, uninteresting story, copy/pasted levels and more DLC than can be reasonably counted.