I see a lot of people use mk7 Ht which I suppose is good for lower lvl bases where the attacker doesn’t have access to betrayal but despite being a massive force, it’s still just one single unit and when used in higher lvl bases, they are easily stolen via betrayal or shot down with planes. I think what’s donated really depends on the higher lvl player but the minimum should be cannon types like howitzers/field/heavy arts. My preference is mixing up the troops usually having heavy rpgs and maxed armored cars. Anything can be shot down with planes but because there’s a speed difference it can make planes miss. Because there are several units, you can’t steal them all and people tend to grab the normal tc defenders so that’s also a factor. In lower lvl bases, attackers can’t barrage them all whereas even heavy arts can be taken down with 2-3 barrages and some ranged attackers. Of course as long as oil isn’t an issue people tend to always go factory units as they move up alliance levels.