The two best CI tools I've ever seen were Gitlab, as its approch was just working good (tm) and Teamcity, as its approach with the UI & Kotlin integration was just nice.

Then I've seen Azure Pipelines, which seem like an unfinished product because it competes with Github Actions, and then there is Github Actions which seem to come from someonw who loves to write k8s yml files (they do what they do, but they're unreadable).