The Atlas 7D is fortunately the most common Atlas available, and you should grab one as soon as you can afford it. Then you should get 2 more - no joke! It is the best work horse mech you can get for your money. I gave my 3 AI pilots one each of these in the run up to the final mission, they are rock solid. I find the AI is better at using Burst Fire AC-20s as they are more likely to hit light mechs than the solid shot. The AI doesnt really aim for the head or CT it just splashes damage around, so BF works best. Put two cheap Medium lasers in the arms, as they frequently get blown off. Fortunately the Torso is solid, so you can hunt for good gear to put in it. Artemis is optional if you can afford it.