most overrated: Mighty Morphin. I never really cared for the cheesiness as a kid, but kinda appreciate it now. Tommy is awesome, but I loved him more as the Red Zeo ranger. I can understand the nostalgia value, but the action, especially from the first season are not well edited, the characters are one-dimensional (yet still somewhat enjoyable), and there are a lot of corny liners. The writing improves over the seasons, but there are stronger seasons. I'm not saying I don't like MMPR, just don't love. The series overshadows a lot of great Power Rangers series, to the point that MMPR becomes all of Power Rangers.
most underrated: Lightspeed Rescue and Turbo. Lighspeed is the nostalgia series for me, and there are factors of why PRLR is great. A badass red ranger, decent acting, great pacing, and though people love Ron Wasserman's rock soundtrack, the music in LR fits the heroic image the power rangers have. the villains aren't bad, the titanium ranger is cool, and the American footage is well done. Turbo, at least the second half, is great. I love TJ as the red ranger, I love a kid taking the role of the hero and not just some bystander/victim (inspiration for kids), love the flashy suits, the zords, and the climatic finale as the power chamber is destroyed and the rangers go on a suspenseful trip to space with an epic cliffhanger. I admit that divatox is annoying, but I enjoy the mix of cheesiness and serious plotline. The only reason why Carranger, the Japanese source of the show saved Sentai was the previous show, Ohranger, lost viewers as the dark villains/plotline scaries people as there were recent terrorist attacks in a Japanese subway.