With Klaviyo we have managed to set up some really great loyalty and rewards programs for our clients without using an additional app in Shopify. Klaviyo's flows are easy to work with and 2-way tagging functionality allows you to easily create new segments and/or targeted campaigns as well as lookalike audiences.

Definitely Klaviyo. Totally integrates, can drive up sell, does not require developer skills, easy to create a template, great analytics . You do however want to make sure it is set up by an expert, and that it sends from a “clean” domain.