Torid for sure. I enjoy the infested aesthetic in the game a lot, and beam weapons have always been my favorite type. I got really lucky and rolled a Torid riven for it right after I got it upgraded:

Torid Geli-acricron 114.8% Cold 180.2% Crit Chance 150.8% Crit Damage -41.1% Infested Damage

It literally only red crits, and on Nidus it is so satisfying to larva a whole room, (virulence augment for crit chance) and just blast the beam into a wad of enemies. Even the gas aoe normal fire does red crits and it hilarious when I blast some gas, then pull things into the gas, and boom red numbers start ticking.

Second for me is Burston, for similar reasons but the mag size is amazing.