Realms of Despair
I highly recommend Realms of Despair (RoD) for all mudders. It is truly one of the best text-based games out there. I've been playing roughly for about 5 years and have never looked back.
In my opinion the best thing about RoD is the rich and detailed areas and belonging in one of the player-run organisations.
RoD areas are very unique. The first time you step into an area is like walking into another world. Everything changes. There is a new plot. All the rooms, mobs and items etc are shaped around that plot to forge a fantastic experience. It is like opening the first page of a new book. There is a lot of mystery and intrigue. It drives you into wanting to venture further, to find out what happened, or to unsolve the mystery.
Then when you finally solve the mystery of the area. Be it through defeating the great Daemon Lord in his lair of fire, or ascending the heavens to unseat the False God, or descending into the pits of the dreaded Underworld to challenge the Dark Mistress. Your efforts will be rewarded with the most rare and valuable eq or the simple satisfaction of having solved the mystery or killed the "boss".
Everyone will be, at one point in their RoD career, part of a player-run organisation. This plays a huge part of RoD as these people will become your commrades and they will be the people who you will run mobs with or explore areas with. In most cases a player will first seek out the guild of their class. Then after some time, they might consider joining a more selective organisation, Orders.
As of current, RoD has 8 orders. Each order is presided over by a Leader, Number One and Number Two. Each order has their own unique theme and headquarters as well as a rich and honorable history. Be it the elusive Ascendere or the elitists Inconnu or even the pure, unadulterated evil of the Baali, theres an Order for the liking of each player.