Downtube batteries are the most popular, as they fit neatly into the pre-existing bottle mount screws on the bicycle frame. Also, being in the centre of the bike, you get really good weight balancing.
The only disadvantage is that you can’t use it if you don’t have enough space (33cm of free length) on the downtube. So for folding or ladies or Y-Frame bikes it is not suitable.
Check out the animation below to see how it fits.