There are some very good ones around Lisle Street and Gerrard Street - the ones that look like scuzzy cafes are often the best. Going during mid-afternoon to take advantage of their dim sum service is a good move - nice, tasty, communal eating. Or get the one-dish rice meals - most of them will do a plate of rice topped with delicious sliced barbecued meat, normally for about �4.50.
Slightly further out but still quite central, Noodle Culture, in The Cut (just up from the Young Vic) is a favourite of mine - again, very cafe-ish but good. HK Cafe on Charing X Rd has some interesting and unusual stuff.
Any really big plush-looking ones with high prices are to be avoided as you generally don't get anything better than the cheaper places but you're still paying more.
I can only imagine Wong Kei's have improved their food, Ward. Admittedly I haven't been for about 15 years but when I was a student this place was beloved by those who seemed to want to be hectored by rude staff, which struck me as a bit thick. Only in the UK could you get people excitedly saying "They're so rude - it's brilliant!" When I went I thought the food was characterless slop. But I heard recently that the staff are now polite, which was thought to be very disappointing and not the done thing at all.