I find
wowjuju to be the best site for figuring out what reps are the easiest to tackle next. Just use the calculator on the right.
I assume you are talking about the guild 'Odins Hammer'. If so, looking at the list of reps they are missing, these would be the easiest:
the violet eye
the scale of the sands
Brood of Nozdormu
Ashtongue Deathsworn
if you have the gold, Darkmoon Faire
If I was attempting this (I'm a rep whore in my guild), I'd recommend 'the violet eye' and 'brood of Nozdormu'. I'm selfish and if I am going to grind for the guild, I'd like to have a chance at some mounts. The first boss for 'the violet eye' rep has a chance to drop a mount. Farming for 'brood of nozdormu' gives you a chance at several mounts (usable only in the raid) and a feat of strength for getting the red mount.