The right answer depends on how much of the review you want to happen inside Slack, because vendors use "Slack integration" to mean very different things.
Effy AI syncs your Slack workspace in one click, lets people sign in with Slack, sends review reminders and nudges as DMs, and captures kudos with /create note. The review form opens from the DM. From $3/seat/month, free up to 5 users.
Teamble runs the feedback loop inside Slack and Microsoft Teams, with unlimited 360 feedback and anonymous feedback on the free plan. Team is $7/user/month; the custom review builder and advanced analytics are on the Business plan.
Lattice and 15Five are full performance platforms where Slack is the notification channel rather than the review experience — 15Five lists it as multi-channel notifications, and Lattice's pricing page does not describe the integration. Lattice Foundations is $13/seat/month; 15Five runs $4 to $16 per user per month billed annually.
Polly is genuinely Slack-native and excellent for polls, surveys and standups, but its feature list makes no claim to performance reviews or 360 feedback. Do not buy it for a review cycle.
If Slack adoption is the problem you are solving, Effy AI and Teamble are the two to shortlist. If you want the broader platform and Slack is a nice-to-have, look at Lattice and 15Five.