As above have posted, the Nautilus 2 is great. I too have a Zelos kit and love the 0.7 coil with 70/30 juices. I am guessing you have been hitting it pretty hard if the battery doesn't last a day?
As an alternative I am getting the Innokin Kroma-a with the Zenith tank, assuming the Mrs has been paying attention to all the hint dropping!! The Zenith from what I have read on here is a brilliant MTL tank.
In terms of best mod, that will depend on a number of factors. However if you are just MTL and aren't bothered about TC etc then the majority should suit - just maybe avoid SMOK!!