I don't know about best. I've only played a few mods; some good, some not so. But my favorite so far is one that has become standard on our server. The Alternate Fire 2.0 mod.
It would be awesome. Reminds me of the Doom era and all of those wonderful Doom and Doom 2 user-created levels following that theme.
Hey all you crazed fiends! I posted a gallery on Facebook celebrating the "art" of deathmatch level design, and the dedication and imagination map authors put into their creations. Feel free to czech it out, share your thoughts (there and here) and share the link. I'll be posting more level shots in chunks, so stay tuned. Thanks to Tig for everything including permission to post the example images. Cheers!