Notwithstanding the electrical equipment and gravel area just outside the station boundary, Winchelsea is one of the smartest small stations I've visited (but sadly I haven't yet actually used a train to or from it!). The local schoolkids seem to have provided some decorations, it has a nice little help point amongst some planters/hanging basket-y things, and the overall decor was neat.
I have also recently been impressed by Marylebone's colour scheme, welcomingly efficient train service and overall cleanliness.
The beautiful restoration & maintenance of Battersea Park's ticket hall, with its lofty roof and airy open space, plus the neatly-laid-out facilities, is pretty smart.
I also find Rotherhithe to be a fairly nice station to visit. Yes, it's not big and has few facilities, but it's clean, relatively quiet and is nicely proportioned.
Bere Ferrers was quite welcoming & very characterful when I last passed by and had a glimpse. Naturally, I liked the old signage and the overall character, and the slight obscurity of the route in its present form.
One of the shabbiest I've ever used is Wanborough - I don't know if anything has been done very recently, but the last time I was there, the station buildings were thoroughly derelict, with peeling paint and rotting wood everywhere. The footbridge wasn't too bad, though, and the screens & TVM(s?) were working OK.
I was also deeply unimpressed by the lack of any warm or vaguely pleasant waiting facilities at Dartford during a rainstorm recently. I know it's being refurbished, but it was really quite putrid - random metalwork protruding from the platforms, missing roofing, grotty stairs & some pretty ugly architecture beyond the ticket barriers. Very little seemed to have been done to make it enjoyable!
Lastly, I was not amused by Paignton's NR station. Lots of areas which felt spartan and/or dirty. The footbridge was nice enough, though.