Looks like someone finally decided to make a decent fav mec poll.
Poll : http://strawpoll.me/6522101
Comments (if you dare) : https://www.reddit.com/r/Dirtybomb/comments/40p95u/favorite_merc_strawpoll/
The results of the poll so far are really interesting and kind of unexpected. I myself though I would see mercs like Fragger at the first position, followed closely by Stoker. I could also see Phantom and Vassili somewhat near the top due to how often they are (mis)used. Surprisingly the top merc of the moment is… Proxy ! Followed by Stoker (expected) and… Kira ? Well, gotta agree that the Orbital Strike is really fun, and Kira’s voicelines are extremely good.
Proxy and Kira at the top. Looks like the waifu factor is strong. 
Results : http://strawpoll.me/6522101/r
EDIT : results changed. Mercs at the top are now (in order) Proxy, Fletcher and Nader.