Got a few that would make the list! Each being situationally the best.
Gyrocopters with steamguns
Just by themselves I find that the basic handgunners are useful being a good screen(Stand and shoot and cheap enough to be expendable) and since they are unconditional battleline they are also easy to include in an army, They also synergise well with buffs and they are easy to buff, just bring a freeguild general and you get alot more dakka on up to 3 units for merely one command point (Although this makes them more unwieldy as they are unable to move and keep their bonuses)
large unit of ungor raiders, they are cheap, get to scout move, slap em into a battalion with 6s exploding hits, make sure they are 30+ for rerolls and ambush them unto the board... they are excellent at killing off support character but also softening up targets for your bestigor (once had a 30 man unit bring a terrorgheist down! they are also great in smaler units for objective play and screening stuff. Irondrakes... Probably the most scary shooting unit in the game when all their tech falls into place... 2 shots each with loads of ways to buff em with +hit, +wound and extra rend. And when fired at a monster a trollhammer torpedo is super scary (Once did 11 wounds on a enemy monster). These lads are probably one of the best problem solving units in the game.
The reason I put flamers and starborne sallies in the same entry is that they pretty much do the same thing, They teleport and light something on fire. Sure, the sallies can do this with their allegiance while the flamers need their changehost.
(Coalesced sallies are also great... although they are abit too fighty for me to include as a "shooting" unit as they excel at both ranged and melee)
180 points, Fortunate son booming on the speakers, Runelord in greywater fastness buffing these lads to 2+ to hit with extra rend. Gyrocopters become super scary against larger units and I suspect we will see more of them in cities lists to deal with a skink/mortek/lumineth/horrors heavy meta. They do work in every city, but I found they are exceptional in Tempest eye and Greywater.
...Cygor? Dispelling stuff causes 1 mortal wound... Enough to kill a lumineth unit leader ^^ More meme-ey than good. Hurricanum... More a support piece... But... Comet of cassadora/chain lightning in the Hero phase and then the storm in the shooting phase makes it a potent dmg dealer.
Also should mention I haven't played enough Kharadron Overloads to comment on their boats. They do seem abit lack luster for what you pay for them though. The 2+MW repeating Rocket Dr. Robotnic Artifact seems fun though.