What’s your recommendation for a free CRM tool for an early-stage Startup?

I am looking for a Free (or low-cost) CRM tool that I can use to manage Leads.
Ideally, I should be able to link it to (or at least import data from) Lead Generators like LinkedIn Sales Navigator, Lusha, Apollo.io , DataNyze, etc.
Do you have any recommendations?
Also, do you have any tips/ guidance on how to best integrate the above tools?