Length Wise? Pirate's Booty (Torgue and Pirate's may roughly be the same length but I feel I finished Torgue sooner)
I feel Hammerlock is possibly middle ground, it's got some neat weapons, the main storyline missions aren't that long at all, it does have some nice "hunt this unique monster" type quests
It just really depends on what you're looking for.
They all have nice and unique weapons, I love my Pimpernel from the Pirate's Booty DLC, but I also adore my Sapping Kitten from Torgue.....Hammerlock has some nice weapons too I just feel compared to the two others it just falls more in the middle...I've yet to get O.O.O though and a Twister so once that happens it may change my opinion gun wise (currently running my mechro the most so the any DLC with the possibility of a good shock weapon may win me over)
GT: Social Sin Fanboys = The bane of any intelligent individual
If u love explosive weapons then BUY TORGUE. It has vending machines with all torgue guns. And one is legendary (lvl 48 tho) and u need to get torgue tokens. But they're fun to get. Torgue is my favorite
I would say Torgue DLC. Hearing Chris Rager in this role will bring back plenty of memories of Mr. Satan, and we also get Tiny Tina back. these alone are worth the investment.
The only downside is that it is still arena based, but not like the Moxxi underdome gimmick. It has its own story, and the battles take place around the DLC. It is also a real pain to go through solo, if you are not a live member it could get really hard unless you have a friend to play co-op with. Mr. Torgue will have your ribs aching from laughter.
If you need any help with the DLC hit me up. just make sure to leave a message with the invite so I know who it is, and don't delete it.