Also - and I realise this might be contentious - but I'd suggest one that takes normal batteries. Mine takes 4× AAA.
With Eneloops (or similar low-self-discharge rechargeables), can have a 2nd set that gets you back up and running in under 30 seconds, and if you get really stuck they're sold in every corner store in the world (heck, throw a pack of Li-FeS2 batteries in the emergency kit, 20 year shelf life).
No worrying about having the right charger cable (commonly a Micro USB, something I don't tend to carry anymore), or remembering to charge the thing lest it go flat right in the middle of what you need to do.