really, there is no ONE best delivery device, because different situations call for different things.
So, here is a couple which I think do really well, given the right environment.
Kanger Evod/Kanger Pro Tank Mini- These are really great all day vape devices, if you are someone who likes to always have a couple different flavors ready to go. I usually have 2-4 of them filled with different juices. They use the same replaceable coil heads too. Use Evods for the majority of juices, and have a Protank Mini or Two on hand for the tank killers.
Kanger Pro Tank/Pro Tank 2-Just like the Evods/Pro Tank Mini, but with larger capacity for those juices you love. Glass, so any juice can be used,and again, the same replaceable coil heads.
-The reason why I like all three of these, is because they are bottom coil devices. Basically no issues with dry hits. The juice is gravity fed into the coils. Plus, with having all three, you have a good choice of capacity and no worry about using tank killing juices.
The Classic Cartomizer-Really can't beat the old workhorse. Hassle free, plenty of different resistances available. Simple, effective. Ive been using Boge brand, and they seem to be working well for me.
the Carto Tank-Really, the Protank handles what the Carto Tank does, but there is a different type of vape to be had from a Bottom Coil like the ProTank, and the CartoTank. Perhaps you like the throat hit better on one device. Probably not a bad idea to have one or two on hand.
The Dripping Atomizer- Its good to have a few of these guys on hand, it makes trying juices easy without having to fill a whole Carto/Evod. Ive even got a bridged atomizer, so, not "optimized" for dripping really, and it works fine. That said, I know now that I like dripping, so Ive got some of the nicer HH 357 Attys on the way.
Then, if you still want to try new ways of vaping, read about and learn how to coil your own RBA/RDAs. Probably start with the RDAs, Rebuildable Dripping Atomizers, and then move into Genesis Style RBAs and so on and so forth. Check out the A7 RDA. It comes pre coiled, so you can try it out without having to coil it yourself, and it holds a decent bit of liquid, so its hard to screw it up at first. Nice and cheap, a cool little dripping atty.