One of the greatest - no - THE greatest.

Doom II is admittedly more expansion than it is sequel, but I (and many others) wouldn't have had it any other way. New content like a handful of new monsters and a single new weapon - the Super Shotgun - but less is truly more as the levels and the gameplay itself are polished to a mirror shine and is more than enough to satisfy. You can really tell that the guys at ID were getting good at what they do in this installment, the levels are so memorable, fun and abstract enough to make them feel unique and a bit strange particuarly the ones on Earth and the early levels in Hell.

It's just you, 32 levels of pissed off Demons in varied Space Stations, City Blocks and in Hell itself, 10 weapons (and a new one, the aformentioned Super Shotgun) and 'totally not' MIDI Metallica, Alice in Chains and Pantera blasting throughout.
Count em, 32 new levels to blast through, 7 new monsters to splatter (not including the ones only present in the secret levels then that would make 9 or the final boss technically making 10) and a single new weapon and WHAT a weapon IT IS.
One stock of treated maple and two barrels of metal piloted by one pissed off space marine - the Super Shotgun.

There is honestly nothing else I can say, the game is absolute perfection. The levels are great fun and abstract as hell, the assortment of weapons are all a joy to use and the Demons themsevles put up just enough of a challenge to keep even the most seasoned FPS fan on their toes. At the asking price of $5 it's an absolute non-question I WHOLEHEARTEDLY recommend this title. Between the crafted to perfection shooting, the abstract and memorable levels and the mightily metallic soundtrack by Robert Prince - there is just so much to love about this game.
Oh and not to mention the modding community has kept this game living way passed it's otherwise expected shelflife making the game near immortal, truly a gift that keeps on giving with the thousands of WADs, Megawads, Total Conversions and other mods available. Doom II is a game that is truly timless. If you haven't played before, I cannot recommend it enough.

With that being said, the DOSBox configuration that Steam has packaged with this is rubbish, download a Source Port like gzDoom or Zandronum and that will easily remedy that issue and add features like jumping and mouse aimming and higher resolution options.