Always so much hate for Asterix Marc, lol.
My opinions for what they're worth:
Asterix
A great little park with real effort to the theming, three solid coasters and a good range of other shows, kiddy creds and flats for the family (including an epic mad house, a brilliant magic carpet and a decent spin and spew in hydra). As for the creds:
Goudrix has a unique layout and is very forceful albeit spine-crunching.
O'ziris is a pretty ride - smooth and fun but nothing fantastic (although plenty of cfers have put it in their top 10, I don't understand this)
For me though, Zeus is by far the stand out ride at the park - oddles of airtime and fantastic pacing - in my top 10
Thorpe
No one could call it a pretty or family-friendly park - the six flags of the UK really sums it up. An ok amount of theming, especially from The Swarm. Five stand-out coasters and the slammer being a flat ride that you won't find anywhere else (people either love it or hate it)
Saw It hurts, the first inside drop is quite good...moving on...
Nemesis Inferno the worst B&M invert I've ever been on. Still, it's a B&M invert which makes it worth the trip anyway
Colossus a solid ride and really fun, but no actual wow factor for most people. I like it, but many find it's heartline roles a pointless gimmick
Swarm great because its still new and Merlin haven't let it rot yet. Impressive, thrilling and smooth, if a little short. Great view, near misses, theming, fire, a great soundtrack - what's not to like?
Stealth Although Swarm is my favorite ride in the park, I'd have to say that Stealth is technically the best ride here - the best launch I've been on hands down (whips KK, Rita, i-Speed) and never fails to get my adrenaline going, but it's far too damn short and a bit of a one trick pony
Alton
Beautiful park, gorgeous countryside (plus you get to see the 'real' UK), oddles and oddles of theming and actual history. You could spend all day here without actually going on a ride. Five stand-put coasters plus 13, Spinball and the mine train which are all brilliant family rides. Plus you have Hex - probably my favorite flat ride in the world, Duel - a really fab ghost train and the Sanctuary - a maze inside the castle building. Plus the family can enjoy views from the cable car, looking round the castle/gardens and number of other rides
Rita Decent launch but doesn't really go anywhere, still fun and re-rideable
Air This cred gets a lot of stick, but I really like it - it's not intense but it's a beautiful experience and the closest thing to actual flying you'll ever experience
Smiler Obviously not ridden it yet but it looks pretty damn intense..but then it's a Gerst. so I'll hold my breath
Oblivion Cracking dive machine, the first and arguably the best in terms of drop. A bit short though
Nemesis My number one coaster - it's worth going to Alton just for this. make sure you ride at the end of the day and make your own mind up as to what the fuss is about
So yeah, my choice would be Alton based on rides alone, but it is still miles from London