It's tough to say really. I haven't gotten around to the fourth installment yet, but plan to soon. Therefore, only the first three games qualify for my consideration for our purposes here at this time.
The original Crash Bandicoot left its mark on me by satisfying something visceral I'd been looking for and hadn't quite found in Super Mario 64: a straightforward, traditional platforming game that was in 3D. With the accompanying difficulty level. This satisfied me very much back in 1996-7. As the original trilogy went on, it got more gimmicky and the focus of the challenges on offer gradually shifted from the heavy platforming focus of the first entry to mainly avoiding or defeating enemies instead. It's tough to say which thing I've found more satisfying. I like my platforming games to focus on platforming challenges. However, it can't be denied that I enjoyed the more gimmicky elements especially in Crash Badicoot Warped a lot, such as time trials, aerial combat, bike challenges, and everything to do with Coco, and also just there being more secrets to discover. At the end of the day, I guess it all kinda evens out for me. I think I like each installment of the classic trilogy about equally well.
Yeah, that's a cop-out answer I know. But it's also my honest one.