Viral is good on everything. Swap [Expel Grineer] for a Faction mod of your choice, or put PHC in for a generic build. You could get a little more damage by dropping Expel for Anemic, but it's better to build Expel + Arcane Velocity.

Paper DPS would prefer a radiation build that uses the higher flat values of PHC, but the status effect of viral offsets the increased damage of PHC, making viral the optimal way to build [Regulators]. Even without increasing the relatively low base status chance, they fire so fast that viral procs are almost guaranteed.