Anno 1404 (took everything that was good in the predecessors and expanded upon it while having neither drawbacks nor any major bugs).
Half-Life 2 (quantum jump in graphics quality, animation, voice and character acting, level design and story).
Sims 2 (did everything better than Sims and is still superior to Sims 3 in many aspects)
Jedi Knight 2: Jedi Outcast (much better graphics and animation, cool story, guest-starring Luke and Lando, but drawbacks because there was no skill point system anymore, nor the choice between light and dark side)