As much as I loved the original six - hell, when my friends and I were playing around when we were younger, I could be Kimberly - I have to say Adam.
As stated earlier, the whole, "I'm a frog" thing was cute. He went from Dinos to Turbo and then came back in 2 separate seasons - In Space to help out Carlos after he hurt the pink ranger (her name slips my mind at the moment), as well as Operation Overdrive for the 15th anniversary team up. I'm not going to deny it, I had and still have a bit of a crush on him.
His actor - Johnny Yong Bosch - is a very successful voice actor and singer. You don't hear much about the singing, but he does it apparently. *had to find his email to contact him for a report*
There are many other reasons why I like Adam, but they can wait for another time.