TF2 was my favourite game back in around 2011, it always felt like you could just jump into any game and have a go without needing too much teamwork.
I think I gave every class a good go (except spy, I could never deal with actually being able to trick other players), top are probably engineer, heavy, and medic.
Engy is a relaxing class only if you play on Capture the flag. The other modes are pretty stressful lol. But yeah, I usually also go for the Engy. Scout if I feel particularly adventurous, sometimes Sniper on 2Fort. Spy intrigues me but I'm just not able to use it, I'm a disaster.
Nothing compares to getting into the flow when you're playing Spy, I love some psychological warfare. Stringing together long sequences of backstabs and hide-and-seeks with the enemy team is so much fun.
The knife that hides backstabbed corpses and instantly disguises you as them is also super fun to use.
I can't help but respect spy mains because they play by far the weakest merc in the game. If your team has a somewhat competent pyro on spy duty they don't get to do anything.
I also enjoy playing engineer, but I would play as a more aggro engineer with the upgrade for the sentry where it can't be improved but builds faster and the shotgun where every time the sentry dies you get mini crits.
I used to play TF2 a lot in my school years, but today I find it too intimidating. I've never been good at competitive games, and I'm even worse in my 30s :D But I like the idea that TF2 is still alive, people are playing, and I could return at some point. Alas, with the alleged development of a new hero shooter by Valve I don't expect them to pay real attention to TF2.
I played so much TF2 from probably around 2014 until the dreadful matchmaking update. Still played a bit afterwards but not nearly as much. By far the most I've played Spy and Engineer, always liked them because they were the most unique. Scout after that.