1:. This topic will just be people noting their own favorite bands because of reasons like "ZOMFG THEY BE SO PROGRESSIVE". Everyone tends to think their band is underrated.
2:. Abba is NOT Underrated. Overrated if anything. I swear to god the music isn't even that damn good, why do I have to keep hearing it EVERYWHERE?!
3:. I think the most underrated bands are the ones that actually try something new and inspire yet never get noticed. Lots of high rated bands have "revolutionary" songs that have actually been done before. The Beatles in particular have actually copied a LOT of things. Don't get me wrong they were excellent - but a lot of credit given to them should have went to others.
Madonna was quite good at copying stuff too. Do some googling, you'll come with tons of songs that were copied from, take a look at the bands and you'll end up finding a lot of new gems nobody ever heard of. I enjoyed it a lot myself.