Hmm. Storywise I loved Timeranger, Magiranger and Gekiranger the best, followed closely by Shinkenger (I'd put it on the list just for the last few episodes, honestly).
If I had to vote, I'd go with Timeranger though, it seemed to have the least "hiccups" in its story, and most of the episodes actually seemed to be going somewhere instead of floating in the ether.