Doom 64 is the 'forgotten' installment in this series, but don't let that fool you - this is one hell of a game!
Doom 64 is more or less classic Doom, albeit run on a slightly more advanced engine. The graphics are still in 2.5-D, you can't look up and down, and the basic premise of the game is still the same - except this time around the sprites are much more attractive, the soundtrack is eerie and atmospheric, and the level design is tougher than ever. The graphics really do look fantastic compared to the pixelated PC Doom titles, and although this style of first-person shooters may have been slightly outdated when it was originally released, I still think this style of FPS gaming makes for the best overall experiences.
In terms of difficulty, this is the hardest Doom game I've ever played. The puzzles are a lot more difficult than ever before, the combat is challenging even on the easiest difficulty, and the final boss battle is INSANE if you didn't collect all of the Demon Artifacts earlier in the game. To top it off, there are also quite a few traps scattered throughout the levels, and the fact that there is no manual save option or checkpoint makes the game all the more challenging. Playing an entire level only to be crushed by the ceiling at the exit can be quite frustrating.
As a matter of fact, my only real complaint with Doom 64 is that the save system is pretty poor for a game this difficult. It's not impossible, but it definitely adds a level of unnecessary frustration. That aside, Doom 64 is a near-flawless game in my eyes. It's a ton of fun, the level design is brilliant, and the spooky atmosphere fits the game like a glove.
In short, Doom 64 is a hugely under-appreciated title, and a must-have for every first person shooter enthusiast. I had an absolute blast while playing this game, and I think most fans of N64 shooters will too.