i'm lucky enough to live within an hour of whatley manor, calcot manor, babington and lucknam park so have eaten at all, stayed at 2 and had a good nosy at the rest.
babington nice but I don't think as nice as the hype. a bit self consciously notting hill which is fine but do you really need that in somerset? not sure. also the most difficult to get to, miles away from any sort of motorway.
whatley manor fab. recently done, quite opulent but modern, incredible food (one mich star and another on the way I should think), great facilities (spa to die for) etc. No hesitations in recommending. And 15/20 minutes off the M4.
Cowley lovely - more traditional than whatley, but done at about the same time so all nice and shiny, fantastic spa, excellent food. Whatley has the edge on food and slightly hipper atmosphere I think.
Lucknam very nice, food second only to Whatley, the most traditional of the lot, also the least recently refurbed so a little bit tired. Has a spa but not cutting edge like the others. Being very picky here though!
So I'd go Whatley 1st, Cowley close second, Babington 3rd unless you want to do the whole notting hill plasma screen thang, and Lucknam 4th....