It really depends on your needs, volume, and budget. Mailchimp is probably the "default" option, but there's also ConstantContact, ActiveCampaign, HubSpot, etc.

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).