Several options exist for the large hardpoint, each with its own strengths and weaknesses:
Autopulse Laser: Low OP cost (for a large weapon), fairly versatile but limited effectiveness against armored targets.
High Intensity Laser: Low OP cost, high flux and HE damage output, good range. Has all the strengths and weaknesses of beam weapons. Best used as support for a kinetics-armed ship.
Plasma Cannon: High OP cost, very powerful but flux-intensive, making it a risky choice.
Tachyon Lance: Fairly high OP cost, good damage and range, quite flux-intensive. EMP effect can be devastating. Needs support against shielded targets; kinetic weapons can run up target's hard flux and allow EMP arcs to penetrate shield.