The most overrated coasters I have ridden are, in no particular order:
i. Shambhala - I must add that I found this to rank lower than The Smiler or The Swarm, it was just so boring and lacklustre in comparison. Just about edges being "great".
ii. Krake (Heide Park) - Was built up as being superior to Oblivion, and with inversions and theming to match. Similar to Shambhala, was far too smooth and monotonous to be epic. Still good, though.
iii. Nemesis (Alton Towers) - Never have, and I doubt never will, understand the excitement around it. I think a lot of it is built up on nostalgia and prestige. I expect if Nemesis was installed today at AT there wouldn't be as much praise. Decent, thrilling ride which fits the bill nicely for AT however.
iv. Stealth (Thorpe Park) - short but sweet. However still astounds me today how people can rate such a short and uninteresting layout as 'the best'.
My top 5, to put this into context, are:
1. The Smiler (AT)
2. Flug Der Damonen (Heide)
3. Saw - The Ride (TP)
4. Taron (PL)
5. Helix (LB)