You'll want a stallion with correct conformation regardless of your mare. Your mare should also have correct conformation. That means good legs with the correct angles that are not cow or sickle hocked, bow-legged or posty, or knock-kneed. Front legs can have toes that point in slightly (straight is best but inwards isn't a bad fault), NEVER EVER outwards. Hind legs should have toes that point outwards slightly, never inwards.
The horse should be relatively level, in QH croup to wither is good enough because I'm not sure you'd be able to find one that is functionally level. The hindquarter on the stallion you posted is very nice, and I like the depth of his girth. His neck is quite lovely. Little aesthetic things like head and colour don't matter so much, but his teeth should meet correctly.
He can be over at the knee slightly, straight is best. NEVER EVER back at the knee, it is a severe unsoundness. Bench (offset) knees are an unsoundness as well but depending on how badly, it may not be a problem or it may be a big problem, so you will need to look at your mare (a few really good critique photos would be great, from front, back and sides) to decide whether you need to avoid that fault or not.
His back should be on the shortish side, not too short but not long either, and he should have a strong smooth loin. His croup should not be too prominent.
You will need to use a stallion whose bloodlines compliment your mare's. You also will need to use a stud whose individual minor faults (ALL horses have them) are different to those of your mare, and the stud will need to be correct in the places your mare is not. The stallion and mare should both be of fairly similar type if you want consistency in the type of foal you are breeding for.
You need to consider discipline. What do you want this foal to DO? Different types of conformation are good for different disciplines, for example a top showjumper is built totally differently to a top dressage horse. A top reiner totally differently to a top WP horse. Different BLOODLINES are typically good for different disciplines as well.
The easiest way to pick a stallion for your mare is honestly to post good conformation photos of her in the breeding section, asking which stallions would compliment her best to breed a foal for your chosen discipline. There are a lot of really knowledgeable people on this forum who will be able to help you choose.