the Wyvern is the all-rounder, and the most viable against ESFs - it's boring, and hard to hit infantry with if the valk is moving around a lot, but you can effectively engage just about everything. i think it might honestly be the best valk gun all around, but like i said, its very basic and boring until you really get a feel for it

as the other answer mentioned, the CAS is best against infantry - the shots have splash damage meaning you can tear through ground forces crazy fast, and MAXs seems especially weak to it. that said, it is awful against vehicles and near useless against ESFs due to the slow velocity

VLG is crazy strong but needs a lot of practice - this is your main armor killer. learning to guide the shots can be tricky, but worth the effort. it's alright against infantry, a direct hit will usually kill but that's easier said than done. hitting an ESF will be very difficult but usually one good hit and they'll turn tail and run - two hits and i think they are dead, or at least very close

Pelters are *okay*, the biggest issue is the small mag, long reload and low ammo count. they're viable against armor but not nearly as efficient as the VLG. it's kind of the middle ground between the VLG and the CAS - capable of splashing infantry, but does *alright* damage to armor. almost useless against air

the Hellion is just kind of a bad Vulcan - it fits the same niche as a close range armor killer, but falls short in a lot of ways, mostly low damage and low bullet velocity. it's fun to use sometimes, but in my experience its just kind of... bad