Since you specifically noted that you wanted a heavy band I recommend this one.

https://www.amazon.com/Power-Systems-Versa-Loop-Resistance-Ultra/dp/B0065VN2P8/ref=sr_1_3?ie=UTF8&qid=1497885269&sr=8-3&keywords=heavy+resistance+band

I have tried buying packs with a variety of bands but I never end up using any of the bands except the heavy one.