For those who have not yet enjoyed the wonderful treat that is Guy Ritchie's movies, Lock Stock and Snatch are fantastic. The former is a bit more accessible - it has some violence, but more of it is implied, and the "good guys", such as they are, never really hurt anyone. Snatch is a lot darker and has more on-screen violence, and some people who are meant to be regarded as "good" do some nasty things. Even so, both are extremely funny, full of quotable dialogue, and have complex plotlines, intricate character interaction, and huge casts. Lock Stock is one of my favourite films - I bought an ex-rental last year, and I watch it every two months or so. I have only seen Snatch once, but I plan to buy it at the earliest opportunity and watch it to death as well.
Oh, they both have fantastic soundtracks, too.