I used to play a lot of MUDs back in the day. The best ones IMO are the Simutronics ones (Dragon Realms and GemStone IV) and Aardwolf. These are all fantasy-themed RPGs and still seem to be going relatively strong. Of the three, Aardwolf is probably the most accessible because of the in-depth quality of its tutorial. Actual roleplaying is probably best in GSIV. If you're one of those people that loves a grindy game, they're all excellent for that. I can't speak much to the community situation nowadays. Last time I looked I got the impression that people tended to be pretty atomized in doing their own thing within the groups of people they know well aleady. However, Aardwolf is notorious for its new-player hospitality which is facilitated by the fact that it has global chat channels, something which doesn't exactly exist in the Simutronics games.