As far as specific shooting elements go, quick and dirty rundown. Note that this shit is going to be colored by Mk2 experiences, but Mk3 doesn't seem to have changed much.
Assimilator - Kills infantry dead at range, so long as it can hit them. Ground Pounder produces a shitload of high quality attacks, but it requires you to set it up with Flare or use Mother's feat to get real work out of it. Lost some utility with the change to LOS rules, but still ignores everything but cover.
Monitor - Decent POW, single shot tech. Likely one of the best crit platforms in the game, given that you should be able to boost it's gun every single turn. Gained some utility in being able to see through clouds, though how useful that is remains to be seen.
Modulator - Cheap as shit, decent defensive tech against light infantry, comes with two guns. The direct current rule should be considered a nice bonus rather than something you try to actually do, it's not worth it 90% of the time. Just take the two guns and go with it.
Cipher - Utility gun. Rough Terrain is nice, Flare on AOE4 is nice, POW6 is less nice, but Syntherion can Hot Shot it.
Prime Axiom - Shitload of shots, threatens infantry and heavies alike. One of the best Colossals in the game, and Mk3 buffed it. If you want to play Mother, you want this, and Syntherion loves it as well(though doesn't have to depend on it).
TEP(Battle Engine) - Got buffed, which is hilarious, because it was in the running for one of the best shooting platforms in the game. It will murder just about any infantry in the game, no matter what they are. Doesn't get to be hilarious about it's servitor placement anymore, which is sad, but yea. There are Convergence players who claim a TEP should be in every list, and while I don't agree with them, I understand why they think that way.
My Synth list, for comparison. This was more or less my Mk2 list, so I feel less need to test it, but it's a dependable, all comers list.