Demon Gate/D!zone/etc

Which source port would be good to play the levels on? I came across a set of these wad comps in a friends storage (D!zone (collector edition), D!zone 2 and Demon Gate)

forgot to add: doomsday was also on the spindle

Well, if you want get closest experience, play them in chocolate doom or prboom+ on complevel 3 for doom1 and complevel 2 for doom2 pwads. Or if you don't care much about compatibility, you can load it in gzdoom and so on. 

Prboom+ on software render if you ask me.

Real thing on DOSBox although to get the frontends like D! and such to work you'll also need 4DOS since DOSBox by default still doesn't support them correctly for some reason.  Procedure is to get all the stuff mounted first and then run 4DOS before you run the frontend.

Also worth noting, a very small number of early PWADs for the original Doom won't play correctly even on vanilla v1.9 due to changes in sector movement and/or boss trigger code; for those using a downgrade patch and running them on v1.2 may be in order, I think some ports (PrBoom, not sure about Chocolate?) might have an emulation mode for this.  I don't know if anyone's compiled a comprehensive list of WADs that have issues (something to go on a to-do list somewhere if not, I suppose--it can be hard to pick up on since it's generally not immediately apparent if there's a compatibility issue or just shoddy map design), off the top of my head VILLAGE.WAD is one such that's affected.

Just use ZDoom.

As you've already heard, but most accurate: chocolate doom/DOSBox

Otherwise, use pr/glboom+. Prboom+ for correct lighting, glboom+ for silky smooth gameplay. Pr/glboom+ both have a plethora of added features and enhanced usability without sacrificing actual gameplay accuracy. As mentioned, -complevel 3 for Ultimate Doom, -complevel 2 for Doom II.

No reason to use anything else, outside of edge cases.

5 hours ago, ETTiNGRiNDER said:

Also worth noting, a very small number of early PWADs for the original Doom won't play correctly even on vanilla v1.9 due to changes in sector movement and/or boss trigger code; for those using a downgrade patch and running them on v1.2 may be in order, I think some ports (PrBoom, not sure about Chocolate?) might have an emulation mode for this.

Only PrBoom-plus (not regular PrBoom). Use "-complevel 0" (or choose "Doom v1.2" as the default compatibility level in the in-game menus and then restart, though you'll need to remember to change it back again when you want to play wads that don't require 1.2 behaviour). As far as I am aware, emulation of 1.2 behaviour is as accurate as it can be - no known desyncs in 1.2 demos (though there aren't a vast number to test with). More information here.

This wad showcases a 1.2-specific mapping trick (correctly emulated in Prboom-plus with "-complevel 0"), but I am not aware of any wads that make use of it.

I don't know of a list of wads that require 1.2 behaviour or that were clearly designed for its specific quirks (if you wanted to research this, I'd suggest looking at DSDA, as those maps that need 1.2 will have "cl0" or "1.2" in the demo exe info - N_A recorded some, for instance). I made use of a 1.2 quirk in my E1M7 demo here, which seemed legitimate as it was the current Doom version at the time the wad was released.

There are also some early wads that require Registered Doom 1.9 behaviour (that is, not Ultimate Doom 1.9). The best known are UAC_DEAD.WAD and the original Aliens TC (though modified versions were made to work with later exes). To get Registered Doom 1.9 behaviour in Prboom-plus, use "-complevel 2". I believe the Chocolate-Doom equivalent is "-gameversion 1.9".