Favorite?
Most underrated?
Worst?
Worst would be Luzon or Java. I feel bad about not seeing some of the more beautiful islands in the Philippines and just being stuck in an ugly, crowded, hot, and pretty dangerous place instead. Same for Java except I've been to a lot of other parts of Indonesia so at least saw the good bits, too.
Underrated/overrated combo: Borneo (under) and Bali (over). Many people seem to have this dream of going to Bali, but I think if you're going to hit an Indonesian (or at least partially Indonesian) island, Borneo is the far superior choice. It's beautiful and interesting, not so built up like Bali is, and has amazing food due to the combination of the Malaysian and Indonesian cultures. Plus it is one of only two places that have orangutans in the wild, which were really cool to see. Finally, while on Borneo you can, in addition to seeing the Malaysian and Indonesian parts, go to Brunei, which is not a place I'd want to visit again because of the headhunters (not kidding) and the absence of alcohol, but was crazy-interesting to see once. Bali just wasn't anything special to me and is way too crowded.
Favorite...so tough. If I could only choose one, I'd probably go for the South Island of New Zealand just for the sheer variety and beauty of the landscapes and some of the fun "extreme" sports you can do there. Iceland would be up there, too, again due to the natural beauty. And Borneo for the reasons above. The only thing NZ is lacking that the others have is good food, so it's a tough call.