[Jamesy]

I have a few favourites but in subcategories.

Theme Park: Alton Towers
Seaside Park: Blackpool Pleasure Park
Zoo: Bristol Zoo (although I haven't been to many in the UK sadly)
Safari: West Midlands (I love Longleat too but West Midlands has more of a funfair and more variety in the main safari, although Longleat is undergoing some interesting expansions these next few years)

Any Sea Life attraction I like too. The Great Yarmouth one has been my favourite out the handful of those I've visited.

On the whole Alton Towers has to be my absolute favourite. I'm a massive enthusiast of the place. Aside from the theme park you also have the resort complex which has loads going on, two hotels, a spa, a waterpark, a mini golf course... and new for this year (opening later this month) a lodge/village accommodation complex and also a tree tops course. Alton Towers is generally great if you want a bit of everything. Only downfall is that it's so damn expensive.