Fragger and Sawbonez is included in pretty much every lineup. However, other characters that are used are based somewhat on map, where you are at on the map and the players playing.
Vassili is used by teams who have a really good/consistent sniper. They typically switch to a second engineer role, Nader or fire support depending on what I listed above.
Bushwhacker is pretty much used exclusively simply due to him having great offensive and defensive abilities and the turret is basically a 6th player when used correctly. He beats out proxy due to higher HP and the turret is a lot better than mines plus he has a better gun. He beats out Fletcher because stickies can be wonky right now and they don’t have much range making the turret better overall in many situations plus Fletcher has worse guns.
Nader is used quite a bit for very specific situations (taking out the generator on bridge, covering the plant on the second part of trainyard etc.). She is pretty effective in quite a few areas and sometimes takes the place of a fire support since you don’t always need full ammo.
Skyhammer is used quite a bit due to his solid weapon choices, ammo and his air strike is essential for some maps (EV). He’s used a lot even on maps where his airstrike is little more than a crappy grenade because he’s just solid and you don’t run into ammo situations with him around.
So, the characters that get used the most are Fragger and Sawbonez (in all teams) and Vassili, Bushwhacker, Nader and Skyhammer based on need.
Aura or Sparks get used when the 1 merc only rule is in effect (otherwise it’s two sawbonez) but I think Phoenix will end up being medic #2 when he hits mostly due to HP + Kek…not sure if the self-revive will make a huge difference. Kira gets a bit of play these days on maps where her beam can be super useful since it has such a low CD and people have learned the best spots to beam quickly. So she’s somewhat in there just like Fletcher is used on some parts of a map due to his stickies.
Phantom, Rhino, RedEye, Proxy, and Arty aren’t typically played as much. Not saying they are bad and some players can and will play them. They just tend to get outshined in the current meta due to the other classes being slightly better options.
I do think Phoenix and Thunder will shake the meta up quite a bit though. Well, I think Phoenix will just end up being medic #2 at all times but Thunder might actually make people decide. I don’t believe Turtle will have a huge impact but that’s assuming his barricade is just a small physical shield and would only be, theoretically, useful at covering a defuser from a long range sniper shot from the stairs on the last objective of Underground.
I can go into depth with a bit more of this if anybody wants/cares. Specifically why some classes aren’t used and why aura/proxy/sparks are just bad choices at the higher end of the competitive spectrum.