There are very few mid champions he cannot counter, tankier champs like Malphite and Cho'Gath require more care in taking down and should be avoided if you have low health, especially Cho'Gath who will just eat you. Udyr is a threat but as long as you play the range game and poke at him first, he is dead meat.

The best way to play Aatrox is just to poke with E and Q after enemies have used their skills that may damage or otherwise incapacitate you. After he is fed enough I like the glass cannon approach and go full damage/life steal and at that point every time your ult is up you can pretty much gank someone or two with relative ease. Just try to get them to waste their skills that pose a threat before you Q on top of them.

If you find yourself in the river and you're outnumbered, something that is fun that I do is run behind the baron, wait for the enemy to come to you and then E through the baron into the enemies who just entered his circle. Then Q out and watch anyone who was already low health die. This is obviously situational but it is hilarious when low health people follow out of greed just to feed you more kills.

Attack speed is preferred over damage early game for the very reason that you can heal and do more damage + farm more in safety with a BORK than you can with the bloodthirster.

The bork will provide faster hits to demolish a mage and do significant damage + it has clickable/activated CC which can be used either to secure a kill, or keep a jungler off your ***. I build BORK and boots first, then Bloodthirster, followed by Zephyr and Statik Shiv for movement speed/attack speed and more damage all around.

Last item should be a hydra or Infinity edge dependent on whether it is team fights or mostly skirmishes between 1-2 players trying to steal your goodies at once. Hydra + ult in a teamfight along with a statik shiv will end anyone nearby quickly and most will start to run.

Infinity edge if you are picking off 1-2 people at a time and the enemy isn't grouping because of the damage you'll deal within seconds will be enough to finish one off almost instantly with just the Q--> R--> E combo alone.