If you've got group reactive and single reactive covered, I'd go for debuffs. More specifically the weakness line or the AC debuff line. If you've got those two covered, I would go for the Mark line. The instant heals might also be a good choice. But I don't use them so often in a group that I feel they need to be upgraded beyond app3 or adept 1. When I look at spell upgrades, I look for what spells I use in most situations.
In my case, group reactive before pull, single target reactive, sign of weakness, AC debuff (if the current target is something difficult), Mark (if the current target is something difficult). So I would choose my upgrades in about this order. Though I'd tend toward single target reactive first since it fits into so many situations. Group reactive is really useful, don't get me wrong. But, there are times when I'd rather burn power on my single target reactive (like during a fight against alot of no-arrow mobs, that have been weakened, all hitting my tank at the same time) In this case, the group reactive just sucks up alot of power and the single target reactive gets the job done just as well. But the two I would always upgrade first would be our single target reactive and group reactive. Just a matter of taste of which one to do first.