Wholeheartedly recommend the super slim Bloog MaxxFusion series, available from the very reliable and fast Happy Vaper. This was my starting point for
vaping a couple of years back and, even though I now have a Twist for when I want something with serious oomph, I tend to use the Bloog as my all-day device and when out and about.
It's a KR8 fitting, and the Bloog is a little pricier than some, but I think it's worth it for the build quality. Go for the 78mm automatic battery (2 of) and you really get a lightweight, cigarette-like feel with plenty of vape and reasonable battery life. I've been using Bloog cartos (very good) but have recently been trying some Kanger horizontal coil cartos from CanVape (much cheaper) and they're working well. Also works well with both CE2 and M3 clearos (CanVape etc). Iv'e tried other KR8 systems but haven't found one with the same quality and reliability of the Bloog. They're just very well engineered.
I've introduced several other people to
vaping with Bloogs, including recently a 63-year-old guy with a lifelong smoking habit and major health problems who was blown away by the Bloog and hasn't touched a cigarette since.
I find they work best with a fair amount of PG in the mix, especially when using cartos. If you like high VG or/and dark liquids, clearos seem to work better for those.
I'm actually a little bit in love with my Bloog. If you buy a kit, it even comes exquisitely packaged (a bit like buying Mac stuff from Apple - you get your first delivery and go 'wow').