Duel arena, it's extremely addictive, and for me the funnest thing to do on runescape. You can have 50m at one point, 3b 1 hour later, and back at 50m in a matter of seconds.
I used to be ranked fairly high in Fist of Guthix, it was so fun running up to people with Blitz+AGS spec and watching them complain about getting like 30 charges.