This is based strictly on second hand knowledge picked up from fellow gamers, usenet, and various websites, so take with a grain of salt (except for HOMM4 which I own and have played):
(ranked worst to best IMHO)
HOMM IV - It had/has potential to be a great game, but it was apparently rushed out the door before the AI was completed. The computer regularly neglects mines and does not judge distances, which allows their heroes to be regularly (and repeatedly) killed by wandering monsters. They have just released a 15 meg patch that supposedly addresses this, but based on what I’ve read, the patch has been ‘recalled’ due to odd anomalies that weren’t caught, which is easy to believe since 3do laid off most of the HOMM (and the RPG Might and Magic 9) staff. Pick it up in the bargain bin, or give the 3rd one a shot as the gameplay is similar, just with less ‘stuff’. In honesty, try out the similar, but very entertaining Disciples 2, one of the best turn-based strategy games recently released.
Dungeon Siege - People either love it, hate it, or get bored and give up on it after 20 hours (never to finish it). While many have compared it to Diablo 2, the consensus seems to lead that it is prettier than D2, but not nearly as fun. It is said that you really don’t play the game, but the game plays itself with you only hitting the healing potion button every once in awhile. There is to be a world editor released in May, though, which may lead to some outstanding player made worlds (but I wouldn’t bet on it because of the next two titles you mentioned).
Morrowind - Many are frothing at the mouth for this one, myself included. The game is first-person, lone player (unlike M&M 9 which has a party of up to 6 characters) which may lead to replayibility. The class/skill/race based system is large and in-depth, allowing for a wide variety of character types. The game boasts 6 novels’ worth of text, and 300 separate dungeons. As long as you have a decent computer (I’m thinking 800mhz/256 meg RAM/64 meg video card, but that is a grab at straws), you should have a good time with it. ALso, this game is shipping with a world editor, so after you’ve played and beaten it two or three times, you can create your own world AND character types for others to play with.
Neverwinter Nights - this has all of us D&D geeks frothing as well. The ability to create your own adventure and then publish it to the net for others to play, or let sit on your drive as a ‘server’ for your friends to adventure in has got everyone excited. It is supposed to be true to the 3rd edition ruleset minus some necessary evils (climbing and swimming skills are out, among other things). While this will ship with pre-made adventures, the treat to this one will be playing others games and them playing yours. May not replace the pen & paper game, but will surely fill a weekday night you are stuck at home.