I've always enjoyed sven coop, but I rarely see a good server by me. Pings usually at best are around 150s. I just like it because you're playing as a team to beat different levels, plus the skins they use are great. Only downfall, having to download each map pack individually via their site, other then that it's great.
I also used to like Pirates Vikings Knights(PVK) Just for the simple fact that all you do is run around with big swords, axes etc, plus the knights have a bowman that uses of course, bows and arrows. It nice since you actually have to learn how to angle the arrows as to hit your opponents from a long distance.