Tried several before settling for JDownloads. I would not say it is the best but rather the best for my needs.
It is easy to config and with ftp uploads - all sorted. Brilliant.

Installed well, works well and looks good.
I found the back-end side intuitive and well designed. Integrates well with Joomla and has all the features that I require.
A quality product.

Small, simple, just perfect for my needs. Code easily changed for visual mating if needed.
Great Mod. Used with JZen bridge joining Zen with Joomla.
Works for me.

Great Product
Great Support
Great Value
It does exactly what it says it should do. Simply.
Initially I had problems due to a corruption in downloading the files however support was fast, courteous and eff3ective.
In my case using Zen as single login, apart from the shop customers have access to restricted forums, galleries and downloads, all controlled via Joomla. Disable Joomla login; make the appearance of both Zen and Joomla similar if desired. I used and adapted the mod 'current user' on Joomla front end to polish it off.
Worked for me, but understand how it works and ensure you can adapt to your needs.