Probably depends on both the party and, maybe more importantly, how the GM works.
But I think, as arakish mentions, First Aid would be up there.
I think think a good amount of players neglect Alertness due to the small returns it give, you really have to consistently develop it over time (will have all the players randomly roll for alertness - then when there's something one of them in particular needs to notice I just have them all roll and they are, mostly, none the wiser to what they might have missed).
Attunement is a possible one depending on the GM. It's costly for non-spell types, but might be REALLY handy if the GM requires you to attune to magic items.