Fixed Pulse Lasers are the most heat-efficient laser weapon, as well as iirc the most energy-efficient. They also have the lowest DPS (though bursts are still very close to them in every category besides energy cost).
The thing about building for pure heat efficiency (unless you're running Kinetics alongside lasers which I personally prefer not to do) is that you need to be expecting to face your target and keep firing for long enough sustained times to nearly run them out of juice in order for it to really beat out higher DPS/higher heat weapons in terms of total damage efficiency. That's often very do-able against large slow AI pilots like Dropships or Anacondas and would be a perfectly valid choice (especially since Pulse has great armor penetration to deal damage to their power plant), but when dogfighting ships closer to your own size, at some point the AI or a player can be expected to boost or otherwise maneuver themselves out of your firing point (or firing cone, if you're wasting the Viper's incredible hardpoint placements with gimbal weapons), which means time for the heat-heavy weapons to recharge before the next time you sweep them with a huge burst of damage.
Personally, I use 2x Medium Fixed Beam and 2x Small Fixed Pulse. I used to use 4x Beams, but to be honest the damage from the smaller hardpoints is so negligible relative to what the medium hardpoints can deal, even with beams, that I think the main focus of those slots should be towards energy and heat efficiency in order to give your larger guns the most uptime, plus the superior armor penetration of pulse vs beam allows the small pulses to still be able to contribute meaningfully during fights against larger ships. I'd even consider it a reasonable alternative to remove the 2x small guns entirely and only run two powerful Mediums (railguns perhaps?) for the sake of weight and heat and especially power, though I have yet to try that myself.
