It's probably tougher to say who's overrated than who's underrated. There are literally dozens of great bands who don't get the props they should; some because they've never quite cracked the mainstream & some posterity has forgotten.Bands I would personally recommend include American Music Club, Buffalo Tom, Thin White Rope, The Triffids, The Replacements, The Afghan Whigs, The Wonder Stuff, Shed Seven, The Bluetones, The Boo Radleys & Gene. I could go on...
Overrated is harder, because for everyone who rates a band there's usually someone who hates them. I think probably the most overrated band ever tho are The Beastie Boys, who seem to be inexplicably lauded by people who should know better.