The scout is best up close. As the name implies, the scattergun's bullets scatter as soon as it's fired, which means the father you are away the less damage your going to do. If your close enough you can even kill most classes in 1 to 2 shots which is awesome for lightning fast strikes. Though don't like that discourage you from shooting from a far. You'd be surprised at the kills you can pick off with your pistol or scattergun.
The sandman can be your best friend. It serves as a okay melee weapon, but the baseballs that you can hit are even better. If you see someone that you can't normal kill without a little luck, you can hit a baseball at them hopefully stunning them, allowing you to run up and blow their nuts off. If you hit them from far enough away you can even render then unmovable. The baseball also cause some damage on contact. It does take away 15 base health away, which can be important, so use it careful.
Also some people will say if you come upon somebody face-to-face you should jump around hoping they can't hit you. Some of the better players can predict your jumps ("Impossible!") Others, will just wait for you to return to earth, and blast you to bits. So save your jumps for dodging rockets and pipes at the last second. Even though you have the ability, doesnt mean you have to use it as much as possible. Cause even though your wacky spazzing may throw off their aim, it might throw off yours too, so find a good balance.
Sniper
Sniper, Sniper,Sniper. if you see one in the distance, it almost a kill already. Since most snipers use a rifle which means they are most likely zoomed in, intent on their quest for headshots, they won't notice you until it's too late. Even though a sniper is the from point blankonly one able to cause headshot crits doesn't mean that you shooting them in the head won't cause massive damage. Simple sneak up on them, and aim for the genral upper torso/ head area, then blast away. most of the time you are able to kill then in about one or two hits. Though snipers with the Huntsman, if they know what their doing, will notice you and just aim where your going to be and shoot you with a arrow, and about half the time, that will kill you dead.
Medic
Since medics are a support class, their mostly focused on their teammates, which means thats less focus on you. Unless they pay attention, but if you did your creeping job right, it won't do any good. From attacking medics, I've learned that they have just enough health to survive 2 semi-okay meatshots, which means, you either have to blast them good, or be ready for a third shot.
Soldier
Since Soldier's are a kind of a utility class, expect to run into them a lot. Which you kind of don't want to do. Cause as soon as they see you, they are going to shoot rockets in a downwards motion, cause you to fly upwards, which then ends with you dying from follow-up shots. Getting the drop on them is important, probably the most important on fighting them. The best you can do besides that is hitting your shots and dodging stuff.
Pyro
These are kind of sketchy, cause they are close quarters too, which means that its gonna get messy. I switch to my pistol and try to lower their health enough so im not in danger, otherwise it depends on the situation. So, like the Soldier, get the drop, shoot and hide. Or run back to your team, cause it's not called Team Fortress 2 for nothing.
Scout
Ahhh, scout duels, my absolute most favorite and hated thing in the game. It's all about jumping and shooting. Jump at the right time, and only when you need to cause every time to flail around in mid-air with your back to them, it's a free shot. You can try and get in close and go for those 102 damage meatshots, or stick to mid range, it doesn't really matter. Though try and push them backwards, don't try not to be on the offense too much, cause they can turn a corner and wait for you to run by guns a-blazin'. Then blast you to pieces, something i like to do a lot.
Demoman
Since they lack a hitscan weapon, rush them fast before they can get any traps laid down, if so, then hang back and harass them. Other then that, just try and keep them from getting a lot of spam aimed at you.
Heavy
Even though the Heavy might look like a problem to such a fast and nimble class like the scout, at close range the mini-gun will kill you dead in seconds. So be very very careful, cause once you start shooting, you don't have anytime to waste. But here is where the don't jump rule doesn't apply, if your right next to a Heavy, you need to spaz around cause even though your jumping around, the Heavy is a huge target, and hard to miss.
Spy
There isn't really a direct guide to combating a Spy, due to the fact that they are very indirect and you only see them for a short time. Since the scout is very fast, that means that it can cover more ground which means that you can see more stuff, like Spys. So try not to get too close and don't let them surprise you. Go out of your way to look for them.
Engineers
My most hated class as scout. Due to their little toys and such, running head-on against a LV 3 Sentry isn't the greatest idea. Even though you can use Bonk!, and try to get pass them, it doesn't really do much in the long run to my opinion. Getting help from teammates is best beat against those. Mini-sentries, they have un-goldy range and can put out a ton of damage per second, including the fact that it takes around 2 to 3 close range shot to blow it up, which is a lot. I hate those darn things, you can figure out how to beat those on your own.
My Loadout
1. The stock scattergun, cause of it's versatility and it has very little faults.
2. The pistol because you can use it to weaken enemies or finish them off, also good for taking out sentries.
3. The Boston Basher cause if your in a bind, and your way behind, and you have a medic friends near, you can grind uber faster.
I just put a picture in to show off my scattergun
Targeting!
Have you ever seen two players and wondered "Which one should I shoot at?". Well wonder no more. This is mostly generic. Also for mostly 6 vs. 6 or 9 v 9 competive as scout.
1. Medics with Uber/Kritz
2. The Kritzee of a Kritz
3. Demoman
4. Jumping Soldiers/Scouts*
5. Heavy/Scout*
6. Mostly everything else.
* Depends what you are playing as and what you'r doing.
Competitive Guide Might Be Made
Coming Soon
Though, it's harder to rush certain classes then others, cause you just get mowed down.