The big beam cannons from Freespace 2. Those were the weapons that truly justified flying huge capital ships in space flight simulators (not that you could ever fly anything bigger than a bomber in the game but it's great that it had it those weapons nonetheless). Watching the energy build up on the turret thinking 'ooooohh SHIIIT!!' before it lashed out and carved opponents into space junk before you could say 'shields up'. This also made taking them on in fighters intense because at any moment, you could get one of their anti-fighter beams right in your cockpit!
And yet every game since then has been hell bent on leaving those beam cannons out. The worst i've played being X3, which has replaced those sinister, pants-wetting, over-the-top lances of destruction with high-powered dribbles of piss that take 10 shitting minutes to hit the target that's already long gone! Not to mention that you could destroy your target and his stream of ionised piss will STILL be coming towards you even after the fireworks have finished!