The choice is yours, really. However, I would advise against certain weapons for mains:
GSO gigaton: while it does have more power than a megaton, it takes up much more space on your tech. You will likely prefer stacking megatons over what few gigatons you can fit on your tech.
Any Hawkeye rail gun: the rail gun was first implemented as a sort of sniper rifle, and it used to really live up to that calling. However, changes to the game over the years made it harder to engage in long-range combat (primarily load distances). The hunter is probably the best of the three rail guns currently in the game, but I would still advise finding other, more destructive weapons for the space.
Hawkeye Zeus: I would never use this on a dedicated combat tech. It’s got good damage, but like BF’s class D laser, it’s fire rate and 1 hit-per-shot behavior kills its effectiveness in combat. It also turns a bit slowly, making it poor for getting circling enemies
Other than that, the other weapons have their strengths and fill various niches, so any would make a viable main.
One other note: Venture’s avalanche missile, like most Venture weapons, packs a little less punch. If used in small amounts, the avalanche will whittle down large enemies. Good if you can build a tech that moves fast enough to circle enemies and not get hit. But of course, put enough of them on a large tech, and you’ll quickly take down anything.
Last note: the Naval gun sacrifices space and instantaneous damage for greater fire rate. The choice to use it boils down to whether you want to decimate enemies in a singular, massive blast or distribute that damage over more frequent intervals.
Battleship cannon is kinda the meta outside of missiles. Sure, it doesn't turn much, but it does so quickly, and the turreted guns in the game are pretty much crippled from the speed they turn. Naval gun kinda makes up for it with the rate of fire, however, which is why I would prefer it just to shake things up. Against fast techs, missiles always come out on top because of the lack of shot leading in the AI.
to put my two cents, everything works, just not to the same extent. Like how Geo Jumper stated, the railguns are kinda not effective enough as there is minimal long-range combat, and GSO gigatons aren't good for their size. The choice is up to you. I personally would go for megaton spam if I wanted cost-effective killing, but if you got money, definitely go for missiles of battleship cannons. Missiles are a no-brainer as there is no current weapon leading unless you are using the weapon aim mod (totally not made by Sozin). For missiles, both are good, but I would also consider using seeker missile pods, basically Avalanche missiles with more DPS and more speed for less tracking. I would also use battleship cannons if you like to see lots of boom without the cheesiness of missile-based weapons. The naval cannon is good, but the turning is kinda bad. Lastly, before I bore you out, the deciding factor is about how your primary adds to your secondary. If you got good weapons that can shred shields, go get a battleship cannon or a cruise missile. If your secondary weapons are one hit wonders, go for more dps, like seekers, avalanches, and naval cannons. You just want a well rounded out tech.