I dont think there is one BEST place. You could get lots of different answers from lots of diferent people.
You dont mention the coast so I guess it could be any place in the UK.
The most obvious place is London as there is something for everyone there and if anything is BEST then London is it, one of the great cities of the world.
Many other cities have lots of interesting things to do like Bristol, Liverpool, Newcastle, York, Oxford, Manchester, Cardiff, Edinburgh and so on.
If you mean coast there are good towns all around the coast - Blackpool, Brighton, Weymouth, Bournmouth, Great Yarmouth, Portsmouth and many many more.
The answer may depend on what you want to do, and what part of the country is easy for you to get to.
For example it is no good us suggesting Edinburgh if you want to go somewhere on the South coast.
More details may help.