Kaiser and Xenon are musts -any sort of +str bonus is very helpful.
Though Paladins reach 100% Blast Crit relatively easily, before that a 3% boost from MM is nice.
Mech's +15% buff duration is also wonderful -it works on just about all skills, even those with cooldowns (though there are a rare few exceptions).
Finally, anything that gives boss damage, ignore PDR, or crit damage will be useful.
Things that may seem useful, though aren't, are %HP/MP bonuses. They factor from base HP only, and take up valuable slots. Usually, especially on Warriors, even something like 10% HP will end up being an effective ~5% bonus. That's a pretty minimal boost, and HP is far from key in bossing anymore (things like status resist and DR ignoring skills are best).
#1 Priority should be link skills, especially ones that add total damage or boss damage.