Pick by maturity
Tiny team, low volume
A shared Gmail inbox or Help Scout free tier. Most early-stage companies do not need a ticketing system.
Growing team, growing volume
Intercom or Front. Both blur chat + inbox + ticketing into a workflow that scales with team size without becoming Zendesk.
Larger team, formal SLAs
Zendesk. Still the default at this scale. Painful to set up; powerful once running.
Engineering-led team with developer customers
Plain or Linear (the new customer support integration). Issue-style tracking that engineers will actually look at.
Common mistake
Going straight to Zendesk because "we will need it eventually". You will not, for a long time. The complexity drag of an oversized support tool is a real productivity cost.