I played D2 forever until I got FFXI now neopets absorbs my time. lol Had a level 80 sorc. and level 75 PLD on D2 I found D1 a bit graphically lacking being as I started with D2 so I never stuck with it.
I have Diablo 1 for the PSX and it is one of my fave games on that system. I also played Diablo 2 and yes it is a lot better than the first one, but I wish they would make another one for the playstation. Check on ebay, you might be able to get a cheap one off of there.
Diablo 1 is old school, I had it for the PSX and PC and was a great game although like booboo said its graphics arnt the best(it is very old). Diablo 2 is still a great game, I'm not sure if people will ever stop playing that game. Totally a classic, its what put blizzard on the map imo. I would definently go with Diablo 2 if I were you, tons of game play there for ya. Most stores carry either diablo 2 + expansion or the battlechest still, so shouldnt be too much of a problem finding one.
Both were incredible when they were released. I fell in love with those games. I'd say for nostalgia, you've gotta play Diablo 1. For multiplayer, Diablo II LoD Hardcore can't be beat.
Here's a list of where you can find the ones you want: http://www.mininova.org/search/?search=diablo There are different combos, such as 1+2, or 1+expansions. You'd probably want the ones with lots of seeds, http://www.mininova.org/tor/857926 this one for diablo 2 looks pretty promising. http://www.mininova.org/tor/714918 here's one with lots of seeds for diablo 1 This is using torrents. If you don't know how to use bittorrents, just download a program called utorrent (there are others, but utorrent is the smallest, best one out there), open up utorrent, download the torrent file, and open up the torrent file in utorrent. It should start downloading. Check out the comments if you don't know how to crack the games As for me, Diablo 1 was awesome, but you can't be the mad cow level in Diablo 2. Hilarious