Had a luc4 that worked well for 20700s. 21700s were a really tight fit. So I went with the golisi s6. Amazon had them a while back for 29 bucks. The nitecores ive had have eventually died out in one of the bays. Never had any issues with the luc. Just wanted a new 6 bay
My first charger was an Efest Lush 4 bay. Still works fine, but only half an amp over 2 slots and just simple charging/full indicators. Nothing really wrong with that, but I wanted more.
I got an Opus a year ago and that thing is a toolbox for batteries. Jon said it's the most versatile and feature rich charger/analyzer LION sells and I'm sure he's right. At it's price point there's still nothing like it from looking around the web.
I got a Nitecore D4 in that bin full of stuff from the guy on Craigslist and that also works fine, except 3/4 of an amp at all times no matter how many bays are in use. And yes REALLY snug for 21700s. I had an I2 that I gave away as part of a birthday present.
Myvpro had a sale a little while back and I grabbed a Golisi S2 just because. (I think it was 14 bucks) Great charger. The only thing I wish is that it allowed you to pick it's charging amperage. It forces 2 amps unless you use both slots. (Man that Opus spoils you for versatility. 4 fully independent slots that you can set to 200, 500, 700, 1, 1.5 and 2 amps though only the outer 2 slots allow 1.5 and 2, But you can have all 4 slots doing different operations and or charging at different rates if you want to)
You know what's worth having? Those 4 dollar single bay Enook USB chargers that LION has. He had them on introductory sale for 2:99 and I got 2 with a battery order. They easily fit 21700s and charge at a full amp with all the protections. I use em for singles for mechs.
EDIT: Now that I think of it, I believe it's the Efest that's super tight for 21700s.