Tate Britain has one of the most varied collections I can remember seeing in a gallery, it's fantastic how you can move from room to room viewing abstract installations, classical sculptures along with work from the greats, David Hockney (adored his huge Bigger Trees Near Water paintings), Francis Bacon, Gilbert & George. Theres something for everyone if you look hard enough!
If you have a couple of minutes to spare, read this short review on Hockney's painting in TATE Britain from the Guardian its really interesting.