I'm going to go with Time Force. Althought it's finale wasn't as explosive as Space or Galaxy, over all the series had the greatest cast of Power Ranger History.
I mean a good number of them could actually act! Chemistry between Wes and Jen was a sight to behold and Mad Max alum, Ransik was also a great character actor.
Not to mention it lead to the greatest crossover of all time...... Time Force wins out from sheer stand on it's own quality.
Space was great cuz it gave the Ranger franchise the much needed reboot and recharge to make it actually entertaining.
And the ONLY reason someone would pick frelling Mighty Morphing Power Rangers is because it was the first and most nostalgic.
Galaxy was excellent cuz it had the highest mortality rate of any PR Series to date.