If it's a first time Hawaii visit, I highly recommend you do Kauai. Kauai is small, feels incredibly tropical, full of beaches, and the mountains and landscapes are diverse and spectacular. Oahu busy.. meh.. good intro to Hawaii. Maui is nice too - very drivable, but feels a bit like a giant resort. Hawaii (Big Island) feels more exclusive, and run down... Landscape wise I think it's the ugliest; has some hard to access but beautiful beaches, but lacks the incredible mountains you see everywhere on the other islands.. unless you want to hike into Waipio or Pololu.
Personally (and I live here), I think the Big Island is the ugliest above ground, but the best underwater - the reefs are the healthiest and most vibrant. Kauai is the nicest above ground
