My faves...
1) MMPR - True, all it was was "Go, go, Power Rangers!" over and over, yet it was the very first theme, and is a classic. Plus the "They've got/know" parts were great. I think this theme song is the only one that potrays the Rangers as an unstoppable force, even moreso than the PRZ theme.
2) PRZ - I'm talking about the full version, not the short one. This was really cool. They took the MMPR theme and upgraded it. The new lyrics were cool, and the music was great, especially after "Go, go, Power Rangers!" part started playing.
3) PRIS (Extended Version) - This is a very cool theme song. The lyrics may not be the best, but they are cool, and the music is great. Oddly, this extended version is as long as the opening opening credit versions of the other themes, making the opening credits version of the PRIS theme the shortest one ever.
4) PRLR - I really enjoy this one due to the music, mainly, thought the lyrics were cool, too. "Better make it a Lightspeed Rescue!"
5) MMAR - Yes, I'm one of the few that likes this one. I don't know why, actually. I guess since it said "Alien Rangers" instead of "Power Rangers," I found it somewhat refreshing. The only real problem with "Go, go, Alien Rangers" was that the final line was still "Mighty Morphin Power Rangers!" Oh well.