Volt is the best bang for the buck, but the best priced ones are Gen1 2012 - 2013.
Even the new ones from #*&?@^ are not much newer.
Do not buy a 72v Volt battery on Ebay scam. They are 18s and 66v nominal. Will not work in a Gem.
Ask around on the forum. See what others decided. As I sell stuff, I can’t give an unbiased answer. I like whatever I have now! 