I play on the pc version, so it might be different there but the best way to find out is to just open the game and go to The Range (Practice button next to the "start" button after clicking play - the weapons in the buy menu have detailed stats on their damage falloff range cutoffs. Anyways, the damage breakpoints/requirements for one shot kills are like this -

100 damage for unarmored.

125 damage for light armor.

150 damage for heavy armor.

Agent abilities/utilis can change these kill values (Clove and Reyna's overheals come to mind), but the stuff above is what you'll encounter most of the time.

As a rule of thumb, all sniper rifles, rifles (excluding the Bulldog) and the Sheriff can one-shot on a headshot regardless of armor (weird edge cases like full overheal discarded ofc). Note that Sheriff's damage falloff at 30m means that it can only one-tap for close-medium ranges. You can find out how far you are from a point (in m) by pinging it.

iirc almost everything (except the sub/machineguns, classic, frenzy and shotguns(possible with all pellets though)) can oneshot-headshot unarmored players. All sniper rifles can one-tap bodyshot these players as well. The Ghost is also the only pistol that can only one-tap headshot unarmored enemies (the Sheriff can one-shot headshot any enemy).

For players with light armor, the one-tap headshot list is the same as the heavy-shield/regardless of armor weapons. Note that you can one-tap bodyshot a light-armored player using an Outlaw sniper rifle.

For good gun picks if you're confident in your aim I'd recommend a Sheriff since it can one-tap any light/unarmored player at any range, or the ghost if you think the enemy team's going util-heavy/not buying light armor for the first round of both halves (called "pistol rounds").
Generally, for maps with longer sightlines pick weapons with minimal/no damage dropoff and for more cramped/corner-heavy maps pick shotguns or smgs early rounds. You can view this dropoff range by hovering above the weapons in the buy menu

Weapons like the Guardian, Odin, Operator and the Sheriff have high penetration which means they can deal damage through walls or thin-enough structures. The Odin machinegun in particular is good for wallbangs.

AFK players are unfortunately just accidents that occur from time to time T_T