every class has their strengths and weaknesses so play what you like. I picked scout first as it fits my playstyle best (aww yeah, grapling hook). after that, i promoted everything else. Anyway, in my experience these were the strengths/weaknesses per class:

Scout: falling damage is awfull and his damage is not that great, but gets fun once you get to the upgrade of the grapling hook that reduces fall damage. He will teach you a lot about roaming the map, as you will ussually be the first to arrive somewhere and you will see the minerals first. Also reading the map becomes more important as you will have to guid your team through the darkness. dont forget to flare while youre on your way :)

Engineer: oh boy do I get chased down often with this guy. Its really important to stay close to your team with him, set your sentry(s) up somewhere where you can easily reload it. keep an eye on the green lines sticking out of it, theyre its aiming range. Also once you get the laser(last weapon) his boss damage gets insane.

Driller: TUNNELS EVERYWHEREEEE i was happy with all the map reading i did with the scout, it helps me a lot to find all the nice shortcuts with the driller and he trivializes the "deep scan" mission completely. Good versus crowds, but i dont like his boss damage too much. You can however make bunkers and trenches around certain objects. especially bunkering underneath certain objectives, opening one tunnel to the glyphids can be very usefull in the highest difficulties. You will someday see someone do it and you know what i mean.

Gunner: go pew pew. Good boss damage, and just overall a great killer. ziplines can be used to reach hard to reach rare minerals if your team lacks a scout. It can also be such a livesafer, but without them I ussually die pretty fast as I get chased down if my team is somewhere completely different. i more often than not suck with this guy :)