Chain #20 and Besieged.
Besieged is really nice and fun. 100s of people are defending a city from beastmen forces. The lag isn't fun but the fact that you defend a city and gains some experience points is really cool. The new expansion areas are really cool too. Lots of new places for high level players to gain merits/exp, and much easier than CoP areas. There are few true sight mobs that are not NM but still cool enough and easy to get by them. I've included a pic of me getting upto chain 20 with a pld in the party, XD. It's easy to get 6-10k per hour. I'm wearing ACC, EVASION, and STR/ATTACK gear, compared to my normal def, vit, and enmity. People can skip CoP exapansion but this is a must for everyone looking forward to level 50+.