Chef and from what I heard, since I didn't use it, Puppet are declarative, but since their DSL is really Ruby, it is really easy to introduce imperative code.
Ansible uses YAML, but when I used it few times it felt that you still use it in imperative way.
The saltstack (which also uses YAML) was the closest from that group (never used CFengine, but the author wrote research paper and shown that declarative is the way to go, so I would imagine he would also implement it that way).
If you truly want a declarative approach designated from a ground up, then you should try Nix or NixOS.
Ansible uses YAML, but when I used it few times it felt that you still use it in imperative way.
The saltstack (which also uses YAML) was the closest from that group (never used CFengine, but the author wrote research paper and shown that declarative is the way to go, so I would imagine he would also implement it that way).
If you truly want a declarative approach designated from a ground up, then you should try Nix or NixOS.