Basically, what the title implies. There's only one rule: No mapsets or mods that are essentially mapsets. As a baseline, the coding/look changes should be at least to the overall level of Spear Resurrection, the DHW TC, and/or WSJ's Dos mods (all of these inclusive). And as a guideline, I'd say list somewhere between five and ten, so there's some room for manipulation depending on how many you can think of. If you aren't sure of absolute favorites, then candidates are fine.
And no particular order necessary. Mine would most likely be (going to list a full ten, as they're all great IMO):
Spear Resurrection (2001, original)
End of Destiny
Coming of the Storm
ATAW3D Floor 69 (I know, it's only one level, but I MUST include it, and there's enough special effects to qualify!)
Guns & Glory 0.5
As a footnote, try to avoid too many Dos-only games because oftentimes I cannot get them to run smoothly enough to be playable (WSJ's Dos stuff does okay though). And yes EoD is okay, as I have played it through, not to mention I included it myself!