Because my count is relatively small, I'll do it by park (ignoring the smaller parks and Drayton because, well, they're ****)
Flamingo Land's Velocity is hugely under rated imo, it's a great family and teen coaster, the seating position is unique, the launch exciting without being too much or too rough, although instead of the three almost helixes with near misses it could probably add another bunny hop or two or 'chicane' bends. It's by no means a top ride, but it definitely deserves more praise than it recieves. Jesus, to the extent that I actually forgot about Flamino Land entirely when filling out Mitch Hawker Poll. D'oh.
Air is under rated in the sense that for what it currently is. Family coaster, step up coaster that provides a thrill to those not willing to jump into the queue for the roaring mass of steel and bloody water next to it. It's market has changed from when it first opened but I feel it currently hits it's target perfectly. I daresay Nemesis is over rated by the UK, although that's not an all to uncommon view as it's one of the best rides in the country, but on the continent it's merely a very good ride.
I'd say Revolution is under rated at Blackpool. Fun ride, fun at the front and the back and a rare experience both in the park (something relatively modern!) and in the country. Not sure which of the woodies I'd say was over rated as my views are skewed by viewing specific boards so I'd be basing it off of their biased views rather than the view of a ride generally so I'll stay away from listing one.
Tomahawk at Port Aventura is hugely under rated, Jesus what a fun little woody that one is.