Put my vote in for a wismec rx200. I love them enough that I have been stockpiling them in the closet so I can have them in case the FDA ever comes down hard on vaping. For around 40 bucks plus shipping, it is a tough mod to beat.
Then again, I am a bit biased. I run a TFV4 at 90-120 watts all day and there is no way I could safely do that with a single battery mod. In addition, the three batteries give it great battery life, even when chain vaping. The only downsides to three batteries are the weight, and that you should ideally have a quality 4 bay(or 3 bay, but I havent seen one) charger so you can charge all the batteries at the same time. That way they are almost always all at the exact same state of charge. I view the charger as a moot point, though. I end up using it for all of my AA rechargeables for my xbox one controller and my telescope. It serves more purpose than just charging batteries for the rx200.
This picture should show where I stand on how much I like the rx200 lol