There are a few I like.
Dingwalls, which is in Camden, in London. It's really small, I think the capacity is about 300, but it's tiered. You can get an awesome view wherever you stand, no matter how tall you are (I'm short, so this is a winner with me).
Not been to a huge number of gigs there, but have seen Candlemass, Union and Evergrey there on separate occasions. The sound is always excellent.
Koko, also in Camden, London. Used to be the Camden Palace Theatre, so inside it looks a lot nicer than most gig venues I've been to. Three levels, each with various heights of standing area, and for some reason lounge areas out the back. Again, always a great view. Seen Celtic Frost, Paul Weller and Hammerfall there. Again, always great sound.
Also like the Shepherd's Bush Empire in Shepherd's Bush and used to like the Astoria in central London before it got closed down. Hammersmith Apollo is also usually pretty good, with good sound.
Of the large venues, the O2 Arena in North Greenwich, London is actually really good. They've done a great job with that place after the disaster that was the Milennium Dome. Obviously sitting at the back of the top tier is a pretty useless view, particularly in the end-stage set-up, but I saw Metallica play there twice in a centre-stage configuration and it was superb. I did have lower tier seats for those. And when I saw AC/DC there I had lower tier seats practically on the stage.
10/12 Black Sabbath
18/12 Black Sabbath