Sickle, for sure. If you pace your fire properly, it's got the best ammo economy out of any primary (outside of the Blitzer, maybe). The armor pen leaves a lot to be desired, but aiming in first-person is accurate enough that you can shoot around armor or target small weak points from afar with relative ease in a lot of cases.
Unless it's a hot planet and I'm expecting a lot of engagements, in which case I'll go back to the Slugger. It feels like overkill for most enemies, but damn is it satisfying to use. I especially love the Slugger's sound design, it's got such a satisfying "kachunk"y feel to it.
Breaker simply go brrrrr. I think I've been sliding more towards the spray and pray, though, since it eats light units and I'm already carrying an autocannon most games.
I know the devs have stated that they want the primary weapons to feel weak but I think they forgot to apply this balancing philosophy to the scorcher, breaker (+variants) and slugger. I use the scorcher myself just because it's more versatile than the others in the top tier but if the game were better balanced I would have been spamming the plasma shotgun, it feels so fun to use but it performs like a direct downgrade to the scorcher
I've started using SG-8P Punisher Plasma. I am a big fan of just exploding wave after wave, though I still need practise with it.
Used Recoilless Rifle stratagem previously. But now I switched to Quasar Cannon which lets me use a Rover to help deal with melee.
Considering buying the Senator for taking out un-armored targets at long ranges.
Surprisingly, the scorcher with the SMG pistol and quasar cannon. A heavy loadout but against bots you can pull down a lot without getting overrun. Friends deal with the little crap haha