More than in most other wargames, the performance of a HQ depends both on the officer in charge and the staff. Even with a mediocre leader, you can quickly get a good percentage modifier by playing Personnel cards.
Based on my limited experience with the game thus far, the Personnel card is on the long term the most valuable High Command card because it gives a permanent bonus (an increase in HQ experience) that doesn't require another card to be played. You do need to be careful about adding replacements to a HQ as it will lower their experience rating. I'd say that as long as the staff is full strength, a HQ doesn't need a full infantry complement for example because it will generally be behind the frontline.
It also seems command points are in a way more important than the other ability ratings, unless there's a really big difference. Keep in mind that none of the abilities matter until cards are played. If you can play a card every 6 turns and it gives a 30% bonus (a theoretical example), or a card every 4 turns that gives a 20% bonus, I'd pick the latter because even though the leader is somewhat worse, he can play cards more often. I'd guess that most if not all of the leaders that give a reasonable amount of command points also have good ability ratings, but still.
< Message edited by ComradeP -- 7/23/2012 3:14:03 PM >
WitE Alpha tester
Panzer Corps Beta tester
Unity of Command scenario designer