i agree sequels in video games usually tend to be better than their predecessors in one way or another. i think the reason for this is obvious, a more experienced dev crew will try to improve upon their previous efforts. good devs know what parts of their games worked best and they know what part of the game need improvement. i think there are fewer instances where they got it perfect the first time around. that being said heres my list of best sequels
sonic 2
super metroid
star fox 64
f-zero x
resident evil 2
pokemon gold and silver
age of empires 2
tony hawk 2
metal slug 2
mortal kombat 2