I still remember my first Doom megawad, I think it was 2002: A Doom Odyssey for Ultimate Doom. At the time I was unaware of source ports so I made a poor attempt to run through Doom95. The first port I downloaded was Doom Legacy and I still think it is an okay port. But now I use ZDoom and eventually I played 2002ADO using it much to my enjoyment. There are so many mods for Doom it is pretty crazy, you will almost never get bored with Doom if you like fan-made content.