It really depends on your needs, volume, and budget. Mailchimp is probably the "default" option, but there's also ConstantContact, ActiveCampaign, HubSpot, etc.
It's also a question of what other services you might need to integrate into (your CRM or sales funnel or analytics or whatever).
Depends on so many things, and there are so many providers out there. What industry are you in and what are you aiming to accomplish? Who are your email recipients / targets?
It really depends on your needs, volume, and budget. Mailchimp is probably the "default" option, but there's also ConstantContact, ActiveCampaign, HubSpot, etc.
It's also a question of what other services you might need to integrate into (your CRM or sales funnel or analytics or whatever).