Best seawatching spot for me by far is Point Lynas, though i've not been there for a couple of years. S Stack is too high, but you can seawatch from the headland to the south locally known as "The Range". Cemlyn, though very low, can be quite good too for seawatching.
Malltraeth Cob and the adjacent estuary are often worth a look.
Newborough Forest also, depending on what you want to see.
Malltraeth Marsh RSPB is a huge reserve which is underwatched and has had several rarities, including at least one Baillons Crake this spring...
Worth contacting Alan Davies who guides on Anglesey all the time:
http://www.thebiggesttwitch.com/