Movement, sensitivity, purchase knowledge, grenades, mental focus and perseverance.
Crosshair placement, movement, mental focus and aim mainly but mostly important imo is learning obvious things. For example if you kill one or two opponents at a/b, there's a big chance there's 0 or only 1 more player there = go for that site w.o hesitation.
1. Spray control
3. Crosshair placement
Though the most important thing is GAMESENSE, but it comes with time & experience
apart from the obvious that everybody already said, I think a skill a lot of people need to learn is when to push, when to be passive, when to rotate etc, best way to improve that is to watch pro games imo
depends what level, i am at a point where i know where to put my crosshair but i can rarely spray at long ranges and that's what i need to work on
dealing with cheaters, trolls, dumb people, baiters, smurfs and boosted.
How many hrs played, what is your fps experience?
besides game sense, which comes with time, probably stuff like crosshair placement and pre-aiming, then aim and spray control, then trading / not baiting, and grenade usage
I mean probably nades, aim, awareness, understand of buying, movement, entrying, patience, when to passive, rotations, map pool.
1-crosshair placement/pre aiming:mine is very good but my aim overall excluding awps isn't that good,but i get lots of kills by placing my crosshair correctly when peeking
2-map pool:try to master maps where you're good,not only nades,learn timings,strats,angles,plays,spots,give a special attention to pre aiming(yprac mirage for example is a very good map for this and learning nades),afterplant plays,game reading and etc,i see it's very neglected by lots of players that just know like 3 nades and that's it but it just boosts your gamesense by far by doing some study on the map you choose
3-economy management:having a better gamesense to know when buying,when forcing,when semibuying,when ecoing and etc is like one of the most important things in the game,but it's not only this,you need to know how to do economy tracking(or atl how to do a decent prediction) to know what kind of guns you're dealing and how to exploit it(ex if you think it's and eco try to put yourself in a position where you'll have gun advantage,like ak vs smg,eagle or cz at long range,and try to avoid short range duels where you have a great chance of giving your gun away and throwing the round,do the opposite when you're ecoing basically and you will be fine,and remember that you don't need to win an eco,if you take 2 or 3 guns or plant it's very nice,don't tilt bcs you didn't win it)
edit:in 3 i'd say that you can improve a lot by watching your own demos to see where you should make an eco and if your predict was right or wrong
first learn how to aim and shoot. spray and stuff...
then learn the maps, with that, crosshair placements and awp spots, possible rush timings what is possible to do, who gest where first... basically maps particularities/spots/where to aim, where does players comes from...
then finnally learn executions, granades, plays.
> USE THE DAMN MINIMAP!!! all the time. get used to it. take advantage of your teammates positions (bait)
> learn CT economics and weapons (no AK)
> learn T economics and weapons (AK).
> build your ECO, half and Full buy gear in your mind.
> Be mindful of your audible sounds and listen to other players sounds.