What's the best tool for understanding user behaviour?

Smartlook is a good complement to GA data. GA give only quantitative data which are good to understand the what. Smartlook provide qualitative data like user recording, which helps to understand the why behind GA data.

Maybe take a look at Hotjar, it allows you see what users are doing on your site.

Amplitude has some automated reporting feature like personas which cluster users based on their behaviour. However those feature are only in the premium plan and required quite a lot of data to be accurate.