Doom is a classic. You can play it as it is or enhance it with the numerous 3d-accelerated engines you can find on internet for free, which can make those square-shaped textures as smooth as actual (well, not the latest) 3d games. But that doesn't really matter. It's an unique experience on its own, no matter if the graphics are dated now and the sound is 8 bit, it's ok how it is. I played Doom on my Amiga, running it at 320x200 and with average framerate of 13/14 fps, and now played it on my powerful pc, at 1024x768x32 with a framerate of 60/85 fps (with Doomsday), with a mod that adds 3d models and objects, and I enjoyed playing it exactly the same way. A perfect game, from gameplay to design.
The only thing I don't really like so much is the music. It can sound good for someone, but I do prefer hearing those mosters screaming in my ears, as those sounds are a classic as well as the game is, and because this way the games looks more "realistic".
I tried the "Ultimate" version only recently, just to find out that it got one more chapter. Well, if you already got Doom I suggest not to try it. The new levels seems to be made from another person or staff. The difficulty level is not well calibrated, so you may find some earlier levels more difficult than later ones, and the design is incomparable to the original 3 chapters. Just a commercial operation to sell some more copies of Doom, or to sell it again to Doom fans. Consider my vote correct only if you haven't got Doom, but if you already have it my vote is 1, as you can find much better levels on the net for free.
My vote also considers the gameplay from a 2006 year's point of view, when you can play it with the "wasd" key setup and a mouse, this way the game gets easier than the original Doom, as you needed to keep a button pressed to strafe (if I remember well) those days. The graphics are considered from those days point of view.