Its all just a matter of opinion. I just couldn't stand most of the main cast. Still, they weren't Operation Overdrive bad, or Samurai bad.
Personally, my favorite series is Lost Galaxy. Its my favorite team and my favorite series (although Tommy is probably my favorite ranger). I'm also a big fan of RPM and In Space. I've watched all of Lost Galaxy, Mystic Force, RPM, and In Space. I'm currently watching Lightspeed Rescue, although I'm going through it very slowly. I'm sort of excited for Megaforce (I'm hoping its done better than Samurai), although I love Gokaiger and wish they had adapted that instead of Goseiger (it would be awesome to see the mobirate and ranger keys be made in the US, plus Gokaiger was just a cool show/concept).