There are way too much Lineage II servers out there allready so that one doesn't get my vote.
World of Warcraft is one of those games that's worth paying for the real server, since you need tons of people to enjoy the game =)
I haven't played Ultima & Rose, but heard Rose was a very good game ...
[sarcasm]Or maybe you could do a Runescape server =) [/sarcasm]