Probs Halo CE tbh. I spent a fair few thousand hours playing its MP, and the non-modded maps had maybe 4 weapons you could actually use, and the netcoding is so abhorrently shit that even having 1 less MS of ping gives you a noticeable advantage.

That said, the popular mods are so broken that it's not even fun anymore. There's this map called Bigass or something that was released several times since 2013 with slightly different states, and everybody plays it because "muh reach reskin," and each match always starts off the same: 10 minutes where absolutely nothing happens because it's so big it takes ages to find an enemy, and then once somebody camps at the cliffs of the enemy team's base with a Tank/Sniper/Gauss rifle/Hog, that team automatically wins as their base becomes a meat grinder. Also the grenades are even worse than vanilla CE and Reach Beta combined, as their blast radius can easily encompass a tank.

Most of the vanilla maps nowadays were modded by some beaner whos' idea of fun is placing teleporters literally everywhere, so most of those servers are unplayable too, but pre-2010 was a fun time fams.