Best engine ever made: HPA. Can do everything at the flick of a switch, from low powered, high rate of fire gun for cqb to high powered DM/Sniper rifle LMG, regardless of the platform. Requires zero maintenance compared to the other existing systems.
As far as platforms go, I would say ARs, nowadays you can find so many different components it's hard not to tailor the gun on your body/style requirements. They also (can) come with ambi controls, so lefties can stop whining
Sights: the ultimate sighting device in airsoft is so technologically advanced very few people have it, yet only a handful of lucky few can properly use one. It's called "iron sights".
No need for batteries, easy to adjust, 100% fine for woodland or cqb, viable for sniping, no ghosting, no parallax correction, no need to spend tens or hundreds on it, no need for ugly looking glass protectors
My personal "ultimate" gun would be a 10.5" or longer AR, powered by HPA, with a good set of irons like my DD replica ones! Basically my Amoebas if they weren't such a pita to HPA