As with a few other people, I'm going to preface this by saying that overrated to me does not mean bad. With the disclaimer out of the way...
Overrated:
Nemesis: looks stunning and is artfully designed, but I always find it a little incoherent after the helix, and too short.
Silver Star: The contrast between this and Shambhala was striking to me - the acoustic screen in the first drop robs the sensation of height, the airtime is decidedly average and the car park setting is uninspired, especially in such a well-turned park. Also, Lewis Hamilton can do one.
Underrated:
Big One: so having lambasted a hyper for not having poor airtime, I'm now going to praise, of all things, the Big One. Why? Because the Big One was never designed for airtime and delivers what it sets out to do extremely well in my opinion. Keeps the speed up well, the proximity to other rides, the sea and the street below still make it feel stupidly tall and the first drop remains my favourite. O also don't think it's as rough as people say, it's just not as smooth as a B&M
Vekoma SLCs: sure, they mostly ride badly, but the first part of the layout up to the sidewinder I actually find really fun and well paced (especially the straight section between the swooping first drop and the first inversion which lets you feel the speed nicely). I do still hate the restraints, mind.
The Swarm: there have been some well-reasoned arguments as to why people don't like this in this thread, but I find it hugely fun, well-themed, exciting and completely re-ridable. On the extreme right seat, swooping low to the lake just makes me happy.