Wow, my last post on this thread was in 2015, and things have changed in 4 years.
Everyone has different vaping needs, which really affects what counts as good/bad.
I am a medicinal user who also vapes w/ friends/family. My priorities are:
1 - Consistency. Every session must work without fiddling or technique.
2 - Efficiency. Weed costs $$$, so I want a little to go far.
3 - Reliability. It's got to work, everyday.
4 - Maintenance. No one wants to spend hours cleaning a vape.
Other factors like flavor, harshness, heat control, convection/conduction were once a big deal to me, but vapes have vastly improved, and most "high end" units work well, taste good, and are adjustable.
BEST: The new Fierce by Healthy Rips. It's a beast that just keeps going. I use it exclusively with dosing pods, which keeps the unit so clean it is almost maintenance free. The only thing that needs regular cleaning is the glass mouthpiece. It's much smoother than my retired Haze Dual too. Very free flowing, and you can hand it to a noob w/out explanation or instruction. I have put over 1,000 pods thru my unit over the past year w/out a single problem - and it still works and looks like the day I bought it.
WORST: The Haze Square. Damn I wanted it to work, and it did, about 10% of the time. So much wasted weed, so much wasted time, on a vape that seemed brilliantly designed, but never really worked right. But as little as it worked, it managed to get very very dirty - and you had to take it apart and clean the 10+ parts and hope you put it back together. I gave mine away.
Honorable BEST Mentions - The Fury 2 is the Fierce's little brother vape, and it works just as well as the Fierce, just in a convenient "pocket size". Also the now-ancient Vapir NO2, a vape so cool it's the one I use when I have a cold, and mine is over 5 years old, and it still works like a champ. (My daughters call it "the black dildo").