Yes the Grand Company leves. The "kill X amount of enemies within 15 minutes" ones pretty much make you an instanced XP camp and with Guardian favor I was getting 500-1k per kill with chains on my 40 Pugilist. They also give you Company Seals which are becoming more and more valuable/useful. The other types of Company leves are good XP too but they aren't as good as that one, if you haven't ranked up to the current top rank you will have a 50/50 chance of getting the kill leves or the defense ones. I personally don't mind the defense ones but your mileage may vary.

If you need gil, though, You should probably stick with regular leves instead of the Company ones, although you could always just farm and your company leves might have enemies that drop valuable items.