I went back and forth between a couple of them -- Kimai2 and InvoiceNinja, back when Ninja was still on v4. Ultimately, every time tracking system does things a little differently, and after switching back and forth again, I've settled back on InvoiceNinja v5 (https://invoiceninja.com/).

If all you need is time tracking, Kimai2 and several others will do the job just fine. But I've found in my line of work that it's useful to be able to produce formal quotes and invoices for tracking purposes. Ninja lets you do all of that, no extensions or modules required, and all of the components are integrated with each other (quotes can be converted to invoices, projects, or both, invoicing can be done by task or by project, expenses can be included in invoices, etc.) and it also features a very nice automated emailing system for client invoice/quote notifications and even a guest frontend for them to log into.

So all in all, I've found InvoiceNinja to be extremely useful and can't recommend it enough.