I'd probably go with the 90's as well. There were quite a few in the 2000's that I enjoyed, but I feel like over all the 90's was more consistently good. The only one I really didn't care for of that time was Carranger and even it has its charm.
Ohranger is my weakest link for the 90s while Carranger is in my top 10 Sentai shows of all time.
My rankings for the 90s would probably follow this path:
1. Dairanger
2. Kakuranger
3. Zyuranger
4. GoGo Five
5. Caaaaaaaaaaaaaaaranger
6. Jetman
7. Timeranger
8. Megaranger
9. Gingaman
10. Ohranger
BTW, for those that haven't seen my reviews, I count the 90s shows as 91-2000. For one, Soda ended his run in 1990, so I feel it works best doing it that way. Secondly, years go 1-10 not 0-9 in my book.
I'd probably go with the 90's as well. There were quite a few in the 2000's that I enjoyed, but I feel like over all the 90's was more consistently good. The only one I really didn't care for of that time was Carranger and even it has its charm.