Doom Collector's Edition contains all the superb Doom titles and comes with a fair Windows 9x port.
The Ultimate Doom (1995) contains the first three episodes that the original registered version of Doom (1993) has, plus a fourth one called 'Thy Flesh Consumed'. This game is my favorite Doom title: all episodes are fun to play, because the well-designed maps and monsters. You can't have all the weapons in the first two episodes (unless you cheat), but your arsenal is enough to kill the legion of demons. Doom II: Hell on Earth (1994) is great too. It does not have episodes like in The Ultimate Doom, but a lengthy sequence of maps up to 32 levels (30 main levels and two secrets). You'll be surprised with the amount of monsters in this game per map, especially because Doom II has several new demons to kill. The well-known Barrels O' Fun (map 23) is one of the best examples of how many enemies you're going to kill in a single map. There is also a new weapon, the Double-barreled Shotgun, which is strong against large mass of weak opponents or extremely mortal in closed-range against strong guys.
Final Doom (1996) has two Doom games: TNT - Evilution and Plutonia Experiment. The last one, Plutonia, is very difficult, especially on higher levels. But both titles are nice for die-hard fans like me. There aren’t new features, "only" 64 new maps to play. If you aren't a serious Doom player, I suggest you to stay away from Final Doom. Period. The Doom Collector's Edition also comes with a Windows 9x port, which does not work very well, after all. Don't get me wrong, I know the difficulties to translate a MS-DOS game to Windows, but they could have done a better work. The 'Demon' monster can actually appear as invisible (at this form, they're called 'Specters'), but if you use the Win9x port, invisibility effects will look fuzzy. What to do about it? . "To make it fair, close your eyes when you're around them.", as the Doom Collector's 'readme.rtf' file says =) . Funny, but also frustrating.
Thus, you can get slow performances using the Win9x port, depending of your hardware (particularly if you're using Windows XP). But I think this is irrelevant in most cases. But the Win9x port works OK in general: You can choose the map, difficult and type of game from the port's front-end.
Doom stills a superb game to play multiplayer matches. And the best: Unlike Doom 3 and Resurrection of Evil exp, you have the choice to play in Cooperative mode. But I still prefer the old deathmatch. You can do both, and don't worry about maps, as you'll have 132 maps to choose with this package. If you don't like multiplayer games, then you have again 132 maps to play alone, choosing between five levels of difficult. Do not try 'Nightmare' unless you're really good!
You can forgive the Windows 9x port minor problems by simply ignoring them or downloading another port (there are several good ones in the web). I really recommend this collection if you're serious about classic FPS gaming.