ZDoom 2.7.1 (Requires a DOOM, DOOM 2, HERETIC, HEXEN, or STRIFE IWAD) ZDoom is an enhanced port of the official DOOM source code to Win32. As such, it not only works with (and requires) Windows 95/NT 4.0, but it also adds new features not found in the version published by id Software. ZDoom may be used and distributed free of charge. No profit may be made from the sale of it. ZDoom was created by Randy Heit using id Software's and various others' sources.
The best source port for modding is ominously here from the enjoyful experience. Today, our number one subject is ACS.
More than just a Doom source-port. I'm including GZDoom in this, since GZDoom is ZDoom's successor these days. A good solution for a retro-styled FPS game, though different genres are certainly not impossible to do in ZDoom.
The only thing I dislike about it is the lack of proper documentation, especially regarding newly implemented features. Thankfully you can ask around on their forums, but it's still an annoyance that should be easy to fix.
best source port, get it on the official website tho
The flexibility that this adds to Doom is astounding. Ramps, 3d floors, prey-like portals, etc... Makes making wads so much more interesting!