Who do you main?

Honestly who you main as Survivor barely matters. They're all pretty much just skins so, just go with whichever one you like the look/sound/lore of the most.

For Killers, I'll describe the play style of the ones I play the most and why I play them.

1. Legion. Legion is death by a thousand cuts. Most of your Legion victories won't be decisive or quick. Your game plan is to keep the majority of the Survivors mending or healing while you slowly acquire downs because your power doesn't really help with that. As Legion you are kind of slowly grinding the Survivors into a fine paste. One other upside of them is that pipping and getting BP as them is really easy.
2. Deathslinger. Deathslinger's play style strongly resembles a glacier. Nothing about what you are doing is fast, but as long as you can aim the Survivors don't really have many options to slow you down either. I usually bust out Deathslinger when I'm sick of getting looped.
3. The Shape or Michael Myers. Be warned, I've seen exactly zero other players play Myers the way I do. The way I see it, Tier 1 Myers is THE stealthiest killer in the game with no contest. He has Undetectable 100% of the time and can swing without losing it. He is only 105% but that doesn't matter so much when the Survivors don't run away until after they get stabbed. However, frequently near the end of the game being the stealthiest stealth isn't what you need, instead you just need as much power as you can get. That's when I start getting T2 and T3.
4. Demogorgon. Demodoggo is best boi. Beyond that... this is another one that I play different than anyone else I've seen. Survivors don't exactly like doing generators while sitting on active portals. I can see them, they can't hear me. Corrupt + 4 Active Portals on top of gens = Best gen defense in the game pretty much. This isn't fool proof obviously, but its really hard for the Survivors to break through and I love it.
5. Blight. WHEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE!

Now if you are just starting out on Killer I would recommend Trapper or Billy depending on if you think you'd prefer an offensive or defensive play style. They're both free, not crazy difficult, and have pretty solid teachable perks.

Survivor: Kate, I have every cosmetic on her

Killer: Bubba, I've always liked playing him even before the buff, it's a great feeling when you can get 3 or 4 down in a single chainsaw (and no I don't do insidious or basement camping)

Meg and Billy. I barely play killer nowadays though

Survivor: Quentin

Killer: spirit and gunslinger

Survivor: Detective Tapp, Felix, and Ace.

Killer: Spirit, Billy, Deathslinger, and Pig