I love Abeno (Holborn) and Abeno Too (Leicester Square). They're Japanese restaurants that specialise in the Osakan specialty okonomiyaki, absolutely delicious food that's quite hard to find in the UK.
If you want a fun experience and like french food - Dans le Noir. It's a French restaurant and you eat in pitch black; it's all about experiencing to food. I'm a big foodie and it was just delicious. They have a great wee cocktail bar and you choose from a selection if menus (courses, drinks etc) but the food itself will be a surprise - so you guess what it is. The whole thing was just great.