I'm an honest fellow, I usually PK around the southern part of Swamp Town near the shops and auction house. There's always a lot of traffic and I have a lot of fun moving around, firing explosive arrows, and planting mines near the shop entrances for poor noobs to look at them for a sec and then step on in curiosity- dying. I don't think there are any big spots in the overworld where players can just sit and "farm" kills. That happens almost primarily in towering, which is why I quit towering. I can utilize my spar and general fighting skills better outside of a small tower room or corridor, and towering never really was designed as a skill game, but more as a numbers game with overpowering the other guild. That's a different discussion, though.
I just have more fun Pking than towering, probably because I don't have a good bunch of friends around to conquer with. I think the towers are outdated in terms of interior design. I don't like corridors or small openings in towers, where people can just sit and spam them so intruders can't get through. There isn't as much needed coordination all the time and it can be quite easy for anyone. There aren't as many opportunities to use any other spar type skill as you'd expect having to literally fight others to control a tower, but that comes down to the flawed design of towers.