Crash 3 for sure.
Really liked the variety and it wasn't as hard as the previous two. I could never finish the first two as a kid but I did manage to beat the third.
Crash 1 is next. A little unforgiving but it paved the way.
Crash 2 just has some unfair and unbalanced levels. Those polar bear sections are awful.
Crash 4 is dead last. Mediocre platformer.
Never played any of the others (except CTR).