Sequels in general don't have a very good track record. "Speed" and "Die Hard" are two of many that come to mind. The originals were entertaining, well made movies. ("Die Hard" remains my favorite Christmas movie, LOL). The sequels fell straight off a cliff. Pure exploitative reaction to the built-in market.
"Godfather Part II" is the classic example of a sequel which holds up to or possibly exceeds the original. But it's a pretty small club. I agree that sequels don't normally do as well.
Speed 2 wasn't good...it didn't suck, but Speed was so much better.
I've never seen Die Hard 2, so I can't comment.
Godfather 2 was awesome, every bit as good as 1. Godfather 3 was WEIRD.