For me it has to be any one of the dark ales brewed by the Dark Star Brewing Company in Sussex, but especially 'Old Chestnut'. More readily available but a very fine ale none the less, would be Harvey's Sussex Best.
I have tried several Real Ales (not all at the same time, honest) and the when choosing my favourites it is difficult.. I enjoy Marston's Single Malt, Black Sheep and any from my local micro brewery - Stewart's (nr Edinburgh).
Probably my favourite all-round beer is Jennings' Snecklifter. The little corner shop right by my flat sells it, which is nice - plus you can pick it up for about £1.30 a bottle from Aldi these days!