What is the best way to track user behaviour?

I recommend Amplitude for event tracking. It's free to start and easy to use.
Once you get that instrumented, I recommend FullStory in order to watch full session video playback of user clicks.

Fullstory. Way easier, automated, you don't need to spend time putting events all over the place.

Individual events are helpful, IMO, if you want to know if people are using a feature at all. But if you want to know HOW they're using it, Fullstory is great.

I think it's free up to 1,000 monthly sessions and they have startup pricing.

I use mixpanel and fullstory.

I'm building NoChurn a set of tools that help with customer engagement to hopefully reduce churn from onboarding and paying customers.

The first tool is a behavior analytics where you track events and daily usage.

Contact me if you find it intriguing / interesting, I'd be happy to talk and demo and get you onboard. The current UI is very ~alpha, but the tracking and daily metrics are in place. I'm slowly talking / onboarding users.

I use Smartlook.com . They have a decent free plan. You can watch full sessions of the users like on a video. It's pretty addictive. ;-) Especially in the first few months after the launch, I would watch first sessions of every paying customer to see how he uses the tool and often I discovered some bugs.

Have used "mouseflow" before, and they have a free tier although you don't get many "sessions" per month.

You embed their js code onto the website on any pages you want tracking, and it will show you video replays of the users' journey from landing on the site to when they fall off.

You can limit the usage, so say you can set it to record every 10th user or whatever so you can make your credits last a bit longer over the month....

Usage which is more interesting you should probably have in your main db
For good UX, there are user recording and heatmaps, user recording are really good but most are paid services that aren't cheap