+1 for Neilson.
The Retreat, Sivota – Lovely location, great wakeboarding on buttery smooth water. Sailing is a little "gentle" at best!.
The Anastasia, Halkidiki – Great hotel, really very good quality. Beach is a bit dull (stones), but they have great gardens and perfect early morning boarding conditions, building to a 4 or 5 of an afternoon for some dinghy fun.
Both have really good bike set ups as well, and are staffed by excellent people. Can't recommend them enough. One plus for Neilson over others is that they include wakeboarding/skiing + tuition as a part of the cost. At Sunsail/Mark Warner, it's around £15 a tow. If, like me, you get in to it in a big way, that can really add up over the holidays!