I’m serious. I know 2fort is like the least-respected map in TF2, and a haven of bad map design and stalemates to end all stalemates. Where a single Engineer can indefinitely keep an entire team of demo-medics at bay thanks to the spawn points and chokeholds all favoring the defensive team to a horrific degree.
But you know what? That’s not what you notice on your first round of TF2. You care about figuring out what this gun is you’re holding. How movement works. Why anyone plays people other than the giant Russian. And 2fort does a great job of narrowing down the infinitely-detailed mechanics and nuances of TF2 into a series of recognizable locations, each showcasing a different mechanic.
Just think of the different locations in 2fort. The battlements are transparently obvious sniper lines, quickly teaching a player about those. There’s a bridge that’s just out of normal jumping range (encouraging rocket jumping), there’s a pool to get the feeling of how underwater combat works, and exactly one alternate path for players who think “I die every time I go across the bridge. Maybe there’s another way into the base?” Even the CTF game mode is the only one that requires no explanation; control points are used differently in any number of other shooters, but a flag is always a flag.
Also, stalemates are a bad thing once you’re actually playing TF2, but when you’re wrapping your head around TF2, it can be nice that the round doesn’t end before you’re ready for it to.
And your second round? Honestly, dustbowl. Due to it being offense/defense and having three different maps it also adds a level of versatility where you could play six different rounds and do something different in each one. It also introduces some more advanced gameplay concepts, especially due to the offense-defense separation. You see the defensive team use Engineers, the Offensive team use Demomen, and both teams use Medics. The Ubercharge and the team push start to show how powerful and important they are, and the spy becomes a presence of mention on the battlefield due to slightly more diverse alternate pathways.
Your third round should be on literally any other map in the game.