Best:
Magiranger - things were set up in the beginning and resolved properly. (eg Wolzard, etc)
Middle to Latter half of Den-O - Hints all over the place many of which were resolved(Mr. Watchman) in the end.
Abaranger - Although there were many comedy episodes, when Aba gets plot-serious, it's serious. The plots for the cursed armour, AbareKiller etc were totally awesome over the series. I feel that it tied up all the plot points beautifully.
Gekiranger - loved the backstories for Gekijyuken, Rio and Jan. The way Long appeared early and such. Felt the plot was spun well over the course of the series.
Worst: Kabuto - Had no idea what the eps were about half the time. Too many dropped plot points which were not elaborated upon.(eg,The hopper brothers) though Kagami's character development was good.