Of the heavy energy weapons, the one I fear most used against me is Tachyon Lance. It is not too hard to defend against if you see it coming and do not have significant hard flux on shield. But, if caught while shields are down or burdened with hard flux, it will hurt and likely knockout some engines. Maybe runner-up would be twin autopulse from Paragon or other similar enemy. If that target cannot shield-tank the burst, it will probably die or take a bunch of damage.
Older 1k plasma cannon was only somewhat intimidating during the 0.7a era when player had better skills (to mitigate plasma cannon's inefficiency), although it did not matter since Mjolnir was all-powerful back then, and big low-tech were the best ships, especially Onslaught. (On mod ships with heavy universals up-front, Mjolnir was a no-brainer option over every heavy energy except maybe Tachyon Lance.) During 0.8, weaker skills made the horribly inefficient plasma cannon impractical (except maybe for SO Sunder thanks to that bug that let fading plasma shots hit for hard flux.)
HIL only hits like a truck if not stopped by shields. If stopped by shields, HIL is horribly inefficient weaksauce that cannot hit for hard flux.
Beam spam needs a force advantage to work well, like Eagle against a frigate, or Paragon against most other ships, or a fleet dedicated to it. What I notice is a fleet that can win with beam spam, wins faster with weapons that hit for hard flux (against major factions).
Speaking of gaps, wished there was a better medium HE weapon. Heavy Mauler used to be it, but not anymore (DPS too low for general use now, if extra range will be wasted). Now, it is Heavy Mortar or bust. (Before Heavy Mortar was added, it was Heavy Mauler or bust, but Heavy Mauler was rare.)
It feels lame when the best option for a medium mount is often a small weapon, like tactical laser in medium energy (when I want a beam), or (before 0.9) heavy blaster in heavy energy.