Marble 1

One of the more underrated doom mods out there. The levels have a lot of detail and great use of 3D sectors, used for railings and other things. Not a lot of mods that I know of has these attentions to detail back then other than KDiZD. I like how the first gun, not weapon, first gun in the mod is a quad barreled shotgun. This might be bias but I don't see any other mods or games give the player a quad barreled shotgun as a starter weapon.

The cutscenes and artwork used for them are REALLY well made. I like its story and the protagonist John Logan. His design looks like a cross between Daniel Garners from Painkiller and the Doom Marine from well, Doom 3.

Speaking of Painkiller, the mod uses tracks from Painkiller, which is a nice touch. Admittedly, there are things in Legacy of Suffering that I don't like, such as the Painkiller/Serious Sam arena wave sections. Not all of them are bad, but the later levels that uses them are the worst offenders. Often spawning mancubi, pain elementals, and revenants that just makes the fights more tedious.
DISCLAIMER: SPOILERS AHEAD
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In conclusion, Legacy of Suffering is a really well made mod and I really want other people to try it out. I would suggest to start on the easiest difficulty, do NOT get fooled by the normal difficulty Ultra Violence, it will kick your ass, and anything higher will probably break your soul. Other than that, I would highly recommend you to play this mod. It can be played on GZDoom, Zandronum, and Skulltag. That is the end of the review, have a nice day :)

Enigma is not exactly my cup of tea; the mandatory secrets are a bit annoying and the confusing layout of the maps doesn't help much. Still, the maps are fun to navigate and they feature some great architecture that lead to unique gameplay encounters. Overall, it's decently fun.

Nothing better than baiting the Bazooka Man into blowing himself up! Obituary doesn't pull any punches; you'll be facing tons of hitscanners and plenty of nasty traps, but you'll get to appreciate the fine work the Möller brothers did with this WAD. The maps are intricately designed with a judicious use of textures, some of them custom-made. The new array of weapons is great, especially the flamethrower, which can shred down a cyberdemon in seconds! The low point might be the plain secret map and icon of sin fight, but overall, Obituary strikes home more often than not.

In recent years, general opinon of MM seemed to move towars negative, moreso than any other classic 90's megawad. It's nice to see it recieve some positive attention and even though I'm not exactly a reviewer, I figured I'd join in. Aside from couple rough maps, it remains a great experience today. Generally good and varied vanilla visuals with beautiful highlights like Maps 05, 13, 14 and 21, certainly helped by some great, really cool custom textures, which fit in with the classic ones incredibly well, much better than any other 90's custom stuff (aside from Eternal Doom, but that's a completely different aesthetic). The maps are quite varied without the whole thing becoming incosistent/all over the place and the variety really helps blaze through the whole thing. In comparison, while I prefer MM2 in terms of individual levels, it has a ton of long or similar maps back to back, which can get tiring and take much longer to geth through. Another aspect of MM I really like is some really, unique and fun design choices, or even whole map designs that you don't see much these days for some reason. The Innocent Crew's maps especially are very creative in fun and refreshing ways. Incredibly underrated mappers. Literally everyone knows this already, but Mark Klem's music is amazing. 

This could've been one of the first 32 level megawads that I played after completing the base games. I've played many more since then, and I can honestly say, for a megawad dating back to 1996, it still holds up brilliantly against a lot of modern wads.    Also @DoomGuy999, about that criticism about maps being coop exclusive... "Most levels were made for co-op play, all levels are playable alone as well."   Not exactly sure what you were expecting after having an explicit statement right there.