Totally depends on your budget and what you're looking for....
The Alice in Wonderland afternoon tea at the Sanderson is amazing - around £38 but worth every penny I think personally.
Cheaper alternative to afternoon tea is any of the ones at the firmdale hotels (Covent Garden, Haymarket, Charlotte St are the best) they're about £22.50, and I go back time and time again.
Sophie's on Wellington st is fab too, decor makes it feel quite cool and edgy, and food is fab (steaks, pasta etc) and they do theatre menus at about £15.
Bodeans as someone else suggested is unbelievable if you like barbeque. Queues are huge though and you can't book! Have to put your name on a list and prepare to wander.
Pollen st social is just off regent st and is now Michelin star but still has fab set menus at about £25 for two courses. If she's a Saturday kitchen watcher Jason Atherton is the owner who is on a lot. They also have a cute thing where when you go in they give you a key, and when you leave it opens a little locker and inside is something for you, mine was a scone and a posh tea bag to have in the afternoon!
Absolute best bargain in town though is Belgo. Just behind Urban Outfitters on Neal st in Covent garden, their lunch menu is £7.95 including a drink and is the best sweet chilli chicken, sausage and mash or mussels you'll find in London. Restaurant is underground with tables in the tube arches, and has been featured on masterchef. Waiters are dressed as monks. Sounds weird but is awesome and the one place I always take people when they come to visit.