I use regular soldiers and veteran AT and tanks. Each company will usually have one platoon HQ with good morale and/or combat bonuses. When in command control, these platoons will perform as vets. If I need a really good platoon, I will buy an airborne unit (if available). They usually come with a multi-bonus HQ, since they are considered the elite units of their armies. This is sometimes a gamble. Occasionally the HQ won't have any bonuses.
Tanks may need to be upgraded to take advantage of optics. Most must be at least regular to be able to use advanced optics and some, like the Panther with the dual range optics, must be veteran. Don't every buy a non-veteran Panther. Even without the optics issues, a veteran tank or AT gun will fire faster and usually more accurately than a regular unit. Getting of the second shot before your opponent can be critical to your survival.