regardless of the company, get more capacity than their ads say you need.
Antigravity Batteries was sort of the first in the lithium jump-pack market, and they make quality stuff. i have a friend with their XP-10 and he's been very happy with it.
i decided to go with another major brand, the NOCO Genius Boost HD GB70. it's overkill for the JK's 3.6L, but all that extra capacity also means it might still work when i've forgotten to charge it for 9 months or need to use it 3 or 4 times in a row without recharging. i bought the NOCO after a previous no-name chinese one swelled up and popped apart!
also, i found that the electronics which allow most of these devices to be safe against reverse hookup or shorting out the clamps to each other also prevent them from working at all if your battery is too low. the NOCO has an "override" button that disables all this safety stuff and lets you try to jump a reeeeeeelly low battery still.
Weego is another good brand from what i've seen, but i don't think any of their models have the override button feature.