I've used a few different set ups and like everyone has said - 510 or KR808D-1. Personally I've settled on the KR, particularly with the new premium cartomizers vapor4life sells. KR is a far more cost effective way to go IMO, but there is a learning curve to using them properly - particularly refilling them. IMO the best way is to leave them open and drip them to keep them wet, but with a 510 you won't accidentally let it run too dry because you will run out of vapor right away when it dries up. Flavor is better on the KR.
My advice would be, if you are over the whole "size and shape of a cigarette" thing, to get a mod with a 510 connection and a 510 to 901/KR adapter - then try both the 510 and the KR. My preferred mod right now is madvapes MiniVapestack at 3.7 volts because it is still small light and pocketable and 3.7 volts is enough for premium cartomizers and low resistance 510 atties. Then, once you've decided on your atty of choice get a 5 volt pass-thru for your 510 or KR to extend the life of your batteries by not using them 100% of the time, and also to find out if you'd rather vape at high voltage.
With a good mod you can sell it easily in the classifieds if you decide you want a mod with a different voltage.
juice is the next thing, if you don't like your
e-liquid you won't like your e-cig. Try a few highly recommended juices. I'd recommend starting with Halo Freedom (sweet smooth tobacco), Decadent Habana Reserve (all herbal), and Vapor4Life Peach (fruity) - they are all great, very different from each other, and will give you an idea of the direction you want to go in.