btw, this is tl;dr stuff
This is highly dependable on the priority on the objective and situation (oh and add the fact that every class exists to purposely counter the other classes). But let's cut it down to either ATTACK or DEFENSE.
In my opinion for ATTACK.
I would have to say the most strategic class to use would be the Soldier for one reason.
The Rocket Launcher - The Rocket Launcher is absolutely spectacular when it comes to taking out a large amount of people let alone the fact that usually most of the time you come across Sentry nests or large amount of people. And adding the Kritzkrieg would then allow to give yourself a huge advantage which literally could bring a heavy to lesser than 50% of his health (Provided he was not overhealed). This would mean you are able to instakill any classes at 125HP by default, (Engineer, Medic, Scout, Spy) and two-hit kill every single class in the game. The only downside is the fact that it has 4 rockets which you would have to use strategically or else it would be a pain since the reloading takes quite an amount of time even enough for a scout to kill you before you could reload 2 rockets to kill the scout. (Provided the Scout is at full health and direct-hitting is not an option due to the fact of his speed, of course this is a majority option)
For DEFENSE.
I would go for the Sniper.
Although he can be easily killed, he is usually not the class people would pick to be exposed in the open-area but instead sniping spots, or places from a far that allows him to pick off enemies easily (Provided your aim is decent) with a headshot. No, infact. You can take down literally classes below 151 health instantly with a headshot, and every single class INCLUDING the Heavy once scoping for long enough. (450 damage via headshot, 150 damage via bodyshot). Put on the Razorback and you are basically safe from the back and would take at least one more shot from the Spy (Provided your health is full/overhealed) with an ambassador.