My new favourite is the 2x18v (36v) cordless chainsaw. Will cut multiple nights worth of firewood into 18" lengths on one charge.
As to brand - whatever brand of other cordless stuff you already have.
I've got a number of Makita 18v tools & am happy with them, but if I were starting over I reckon I'd go for Milwaukee - I used a neighbours Milwaukee impact driver & it just seemed stronger/heavier duty than mine.