How far are your commute and other rides you would be doing? I would go with the smallest battery that'll get you where you need to go and be able to handle powering your motor, anything bigger will just be extra weight. And why go 20MPH when you have the power to be going 25 or 30?
When I built my bike, not many options for good pre-built batteries existed. I went with my Ping LiFePO4 because they had good reviews and if something went wrong I wouldn't have a chemical fire on my hands. I also wanted enough range to be able to do all my commuting for a typical day on one charge. It's a brick and the worst part of my ebike experience has been trying to find a good way to bring its fatness with me. Fast forward to today and there are several options for 18650-based batteries that are pretty safe and better options for chargers. Right now I would go with a
Mighty Mini Cube battery and a
Cycle Satiator charger (which I do have) for my current needs. Less than 8lbs and I could go around 10 miles at top speed and recharge in an hour and a half.