Salesforce is the flashiest and biggest. They’re also very expensive, and generally not a great fit for a small business.
Hubspot is pretty user-friendly, and is great for managing large email lists. It offers a free version, though a lot of its most powerful functionality is also fairly expensive.
“Keap” by Infusionsoft is small-business friendly and fairly simple, but it’s expensive and (in our opinion) not very powerful or well-integrated.
Pipedrive is stripped-down and simple. Not too pricey, but again, lacking in power.
Spoiler alert: we love Zoho CRM. Very fairly-priced, but easily as powerful as Salesforce. It can be stripped down for simplicity or built up for complex workflows.