On a training dummy keeping CotE up and nothing else, I tested each pet's DPS for 5 minutes and got the following DPS results:
9100 - Shivarra
8250 - Observer
9600 - Fel Imp
From my testing, it looks like Observer is pretty much last by far, and imp is actually the best. I'm not sure how this might change with different haste/crit/mastery levels, so I'd be interested in what other people find.
Note: I made sure to turn off all specials other than the basic attack special (felbolt, tongue lash, bladedance) so they didn't waste energy.