I would recommend Racket as a good intro to Lisp programming. Powerful system, easy to get started with, etc.HtDP2e
may be a good guidebook. Its geared more towards people new to programming, but that should make it a quick read for you.
Common Lisp has more of a production language feel -- it is big and a bit ugly, but with practice you start seeing how everything fits together to form a powerful system.
Its the dialect with the best commercial support.
Clojure is a good language, but it is essentially tied to the JVM and feels a bit less lispy.
Once you start getting experience with one Lisp system, learning and migrating to another shouldn't be so hard.
What type of programs do you like to write?