In no particular order...
Paddington - bold and awe inspiring, you can really feel the spirit of Brunel there, made even more stunning by the recent roof restoration.
St. Pancras - not only is this a beautiful restoration, but one that also happens to be the gateway to the European continent, it can only get better with the future arrival of DB, etc.
Manchester Piccadilly - welcoming station for a welcoming city, ticket gates haven't taken over, lots going on train movements wise, variety and frequency.
Islip - beautiful little station on the OXF-BIT branch I used to use regularly, lovely single platform in Oxfordshire countryside, wooden sleepers remain in place, the approach from BIT is very straight so you can see approaching trains flickering away in the distance, all due to change with Evergreen 3 and the reinstatement of the Varsity Line.
Minehead - the beautiful seaside terminus of the West Somerset Railway, the old branch from Taunton; as with all stations on the WSR, it is wonderfully restored, has kind and knowledgeable staff, there is always plenty of rolling stock, and the loco maintenance sheds are right opposite the platforms.
Bristol Temple Meads - that amazing train shed; can only get better with plans to restore and reopen Brunel's terminus building.
Hull Paragon - architecturally impressive while avoiding the frenetic feel that can sometimes accompany other stations with similar frequency patterns.
Cambridge - neo-classical architecture and one of the longest platforms in the country.
LUL Westminster - the stunning integration of the station and the foundations of Portcullis House.
Westbury - I like to think of this station as the 'Ely of the west', one of those lovely bustling rural interchange stations that Beeching didn't get his hands on.
I hope that in the next few years, Reading and Birmingham New Street will go from being certs for my top 10 worst to contenders for the best 10!