It is very rare that I login to GTA Online PC and there isn't a cheater on the server. Fortunately they aren't always griefers anymore.
It has been a long time since I saw UFOs all over the place, or random vehicles dropping down from above my head. It is still fairly common to have it raining money bags.
Now randomly blowing up, or someone using a unlimited, no cool down orbital cannon is still very common. I find the cheat that forces you out of any vehicle especially annoying, but the most common one I see used is the invulnerable vehicle.
It is annoying when someone comes and kills you, then keeps killing you and there is nothing you can do about it. This one guys killed me, kept coming after me, I went and got my jet and hit his vehicle with at least 12 direct missile hits. There were a few other misses, but this was a vehicle which may have taken max 3 hits to blow up. Or the guy coming after me in his jet, slams into the ground, skids along it almost complete nose down, runs into a building, getting stuck in it for a while, then manages to wiggle out and fly away.
Sometimes it is hard to tell if someone is using a cheat, because there are some really good griefers that aren't cheating, they are just really good at the game. I have run into some of those too, but those are few and far between.