A General rule of thumb that I follow for the Military skills is this; Choose your military skills to buff your weaknesses.
I understand that choosing your military skills is completely up to the user in it's method of game play. With that being said there are TONS of options out there for easy customization, so let me put my two cents in for your consideration.

1 If you're building toward an AG tank driver, consider your options. Sure you could build it and put ALL your points into the Defense column of the Assault category, but why? Your AG already has Massive armor on it. So maybe add speed? Nope, most AGs are also rather fast for what they are already, with, for example, the SU76i going 48 with no buffs at all.
So how could you build it? Consider your option, As an AG you rely on your turning to keep your frontal armor, so invest in for example, the Immunity research, which will give you some less time off of your Indisposition debuffs that are cast on you. Also consider getting to the skills that give you resistances to the Indisposition attacks. Grab the Ability to cap buildings better, heck you can stand the pressure of being struck by shells, so be the flagger of your group.
Add Skills that will give you more ammunition, more health, better damage. Even then consider that since you are a "meat shield" of sorts for your team, perhaps invest in support skills to help your team out.

2 Tanks in regards to military skills. The main force, the assaulting power of the game are the tanks. The varieties are amazing and many have plenty of uses. So consider what you want to do with your tanks if you want to run with just them. You can build a power house, you can build an espionage monster. Consider using Scout abilities, Rust your enemy, crush his defenses and get that powerhouse sneak attack off because they can't spot you while you're standing still, and can't believe that you're going so fast for being hidden as you coast past the enemy lines.

3 TDs and SPGs share something in common; their defense and health and ammo is low, so perhaps invest in the ammunition, health, and then go down the Defensive column and get to where even the smallest armor items will be more effective than the rest. Fortify research is your best friend. Just remember, fortify is nullified if you move, and takes a bit for the cooldown time. 

Another possibility for the TDs and SPGS. Since you have the range, and are naturally behind the line of battle, consider being a support for your ally. Use that buffed recovery on the tank that just got a nasty hit and needs to be repaired. Grab the AoE effects that buff your allies all around you, they WILL appreciate it.

1. Assault guns are armor monsters, So stack that armor on! Add a health block or two. You can effectively be a fast moving wall, with enemy slamming you for a mere 10 damage on the frontal armor. You are a juggernaut, so build your AG to take those hits even better, you could even add a few catapults or Light armor, to increase your speed as well.
2. Tanks once again have a variety of uses. If your tank is a powerhouse with it's cannon, then increase the reload and the damage output of it. If it has a higher armor count than other tanks then add some more armor to it, add some health to it. If you have a tank that can go fast, then why not buff it to go REALLY fast? The T-34 for example goes normally 55km/h, so slap some catapults on it and WOW with Acceleration2 you are now going 81km/h and 108km/h with the Oil can.

3. Tank Destroyers are damage monsters, so increase the amount of damage they do, and the DPS(damage per second) by adding muzzles to them. Give them the ability to slam for 1000+ by increasing the damage potential, right? You might also consider giving them some more ammo with 1 or 2 catridges.

4. Self-Propelled Guns are a bit different than the rest; Your advantage is range. In fact, your range easily outperforms ANY of the classes. With that being said, no need to buff your range, you already have the advantage. Instead consider this: Add some damage and reload to that monster, and perhaps some more shells. Maybe add some speed to it so that you can move around a bit faster. Remember, you're the sniper of the group, and snipers need to strike with power and precision, so add that power and get out there.

Welcome to where you get to test how well you've built up so far. This is where you show the others that you can compete and pull your weight around. 

So how do you do that successfully? Easy, just remember a few things.

1. Get in a company. No matter what you do withing a company, you will get Bonus exp. You kill a tank, extra EXP. You capture something, extra EXP. Don't sit around by yourself, this game isn't about the lonewolf.

2. Stick with the pack. You ever wonder how people level up so fast? It's simple, they stick with the pack. If the main group is assaulting something, then don't go off by yourself. By staying together there is a higher damage ratio AND more targets for the enemy to hit, so the chances of you being hit as much are greatly reduced.

a. While with the pack, hit as many new enemies that are in your range as you can. As they die you get experience for just landing a hit on them. With the amount of DPS that is being produced from all those cannons, you can easily get experience with that method. I'm not saying for you to just shoot one shell and forget, but rather, if you see that an enemy is being slammed by your team, then move on.

b. Remember, teamwork is the key to this game, and even if no one realizes it, but while in that pack you guys are essentially using a Cantabrian Circle maneuver. Keep moving within the pack, Fire your shell and back off, then reload and repeat. The less time they have to land a hit on you the longer you will survive.

3. If you decide that this is not working, and that perhaps you see a weakness in the enemies defenses, then exploit it, BUT NOT BY YOURSELF. Get your company to come with you, sneak behind the enemy lines. Remember, you are more powerful as a team than by yourself, so always stick together!

-------------------------------------------------------------------------------------------------------------------

Finally, HAVE FUN! If the game gets to be too stressful, then get up, get a drink, go to the bathroom, go outside and get some fresh air, and calm down. This is just a game, no need to go crazy over it and Rage about it. Enjoy the game for what it's meant to be: Entertainment.

I hope this guides helps you guys, and I'd love some feedback.