Ask HN: which NoSQL database engine is better to use with Common Lisp?

Then use Redis (perhaps via cl-redis), if that's what you're comfortable with and it meets your use requirements. Most of the major NoSQL databases have CL bindings. Most of them are also easy to use. It really depends on what your specific data persistence needs are...? All of the db engines have different strengths and weaknesses.

There's cl-mongo (MongoDB), cl-tc (Tokyo Cabinet), de.setf.cassandra (Cassandra), chillax and clouchdb (CouchDB), etc. etc. [Some are more complete and performant than others, but you can find a usable CL binding for almost any NoSQL db. Use a search engine and/or ask on IRC or comp.lang.lisp for pointers.]

That surely depends on your application's requirements?

NoSQL is a broad category containing many data persistence strategies and access characteristics; there is certainly no single tech that will fit every application best. I'm not any kind of cl pro, but the first step is always to work out what performance and persistence characteristics you need.