My Mechromancer is balanced as far as skills but I especially like Little Big Trouble. Considering that, I personally use the Flame of the Firehawk. Mine's level 50 and has decent shield amount (45K) and the radius is nice at 1500. Mine does 72K fire damage from the nova so it goes well with all the shock damage I do (especially when I consider the Electrical Burn skill).
The Transformer shield is cool in that you can take shock damage in order to recharge ur shields. I use it on my mechro. Fire a thunderball fists (or something else which causes shock explosions) at the ground, stand over it and BAM full shields.