Scheme is the best pedagogical language, primarily because it is simplified specifically so that it is easier to teach, while still retaining all of the power of Lisp. It also has the absolute best computer science textbook, The Structure and Interpretation of Computer Programs.

Common Lisp is the most practical Lisp, because it's actually designed for building large systems and for supporting all possible programming paradigms (procedural, functional, object-oriented, logical, etc). There are also a lot of great books written about it. On the other hand, it's a much larger language with many more parts, so learning it is not as easy. You can still get started with simple programs pretty quickly, and gradually learn the large standard library and the more complex features over time.

If you're at all interested in physics and you've read SICP, then you should also read The Structure and Interpretation of Classical Mechanics. This is a very advanced book, but it uses some really clever language extensions present in MIT Scheme to do both numeric and symbolic calculus in order to both teach and implement physics. MIT Scheme is worth checking out if you're interested in advanced programming language design, even if you're not particularly interested in physics.