The Olight UC and Nitecore LC10 are as small as it gets. I own and can recommend both. The LC10 works as a power bank but you’ll need USB adapters because the output is USB-Micro.
The Olight has the advantage of charging Ni-MH cells, although the charging current is “high” for such cells. But most of these chargers do not charge Ni-MHs so the Olight is still a better option
I take along a UM2. It’s a double-slot… but frankly, still small while having a useful display. You do need to have a Micro USB slot power supply capable of delivering 3A. 1A or less and the charger’s display freezes up. If I’m going to stick to mostly 18650 cells, I’ll bring a rechargeable light (Skilhunt H03 w/magnetic charger cable) and have it serve as my charger.
I have that little Olight magnetic one....it's ok, super slow (max 750mAh if the port supports it....if it's a USB 2 port or otherwise hobbled you'll only get 500mAh if you're lucky). With the latter and a 950-ish 14500 cell it still takes an hour and a half or so.
I wanted a little USB powered charger recently as well and got the Xtar SC1 which will do max 2A....however it won't do NiMH and it also won't take AA/14500 cells, which I didn't realize until I received it.
The similar Klarus and Folomov and Keeppower and a couple other chargers had their limitations, too (I wanted 21700 capacity and NiMH). When I looked at the Fenix I could have sworn that it wouldn't fit 21700 but the listing I see now says it will...good option for a quick buy. The Xtar MC1 Plus was only 1A and wouldn't take 21700 I think.
Ended up with the Astrolux MC01 from Banggood and it checks all the boxes and was only $7. I don't know just how "smart" it really is, but it seems to do a good job and the cells end up where they should be. It has a button-selectable current of 0.5A, 1A, or 2A, takes from about 34mm to 77mm in the slot. NiMH is 0.5A only (good, if not slow for some cells). USB micro connector instead of C, plus the 1A type A output if you need to charge something else from a full cell (does have parasitic drain like most so don't store a cell in the charger). Has constant blinking green lights/bars, if that's an annoyance. I didn't see where HKJ or anyone else tested this charger so hopefully it's not actually crap but I've used it a few dozen times over the last couple months, seems like a good all in one single bay. (Also...odd, but it comes in a rather generic printed marketing box. Doesn't say Astrolux anywhere on it but it's printed on the charger).
Edit: just looked on amazon to see if the Astrolux happened to be listed there...nope, but there's an Imren K1 charger that looks identical for around $14, a few different listings for it. Here's one: https://www.amazon.com/dp/B0946JRWP4/
Had I realized the Fenix also checked the boxes I probably would have just gotten that and paid double instead of going to Banggood...
Definitely works! Pretty handy little charger. You'll find that on button tops the end you place on the positive will sit crooked...that's fine, and even if you get it carefully positioned flat it will usually kick itself wonky at some point. Sometimes it will give you the red-green blink and if that happens then usually just disconnecting and reconnecting will fix it (same if it doesn't seem to start...take a little bit normally but if it's not going, just redo it).