My verdict: the king of cold email for agencies.
Smartlead is designed to manage multiple clients. If you're an agency, it's indispensable. Otherwise, look elsewhere. The best tool, but ONLY if you're an agency that needs to manage many client accounts in one place. For this use, it's the undisputed king: white label, ultra-powerful API, smooth multi-client management. However, for internal use on a single company, you can find better and simpler elsewhere.
What I like less: if you're not an agency, the multi-client features are useless and the tool can seem overcomplicated compared to Instantly or Emelia.
My advice: if you're an agency managing cold email for multiple clients, Smartlead is a no-brainer. If you're just managing your own prospecting, start with Instantly (simpler) or Emelia (French). Smartlead becomes relevant when you scale to multi-clients.