Depends on budget and requirements, but, I use the ipv2, it's tried and tested, a good all rounder, and it handles anything I throw at it and I use the following,
Nautilus, runs at 1.8 ohms and 9w ipv2 will run em all day chain vaping with no trouble on one decent battery,
Atlantis, runs at .5 ohms and 22-24w again I get hours out of a decent battery
Lemo, single coil built at 1.4ohm, 14w runs for hours,
Magma duel coil built to .4 ohms, running 30w this is where you need to start making sure you have a good battery due to amp draw etc, I have Sony vtc4 and Samsung 25r bats, the Sony gives me a good 4-5 hrs dripping,
Plume veil, same as the magma,
On top of that, the ipv2 has a USB plug so it can be used to charge a battery too