SBCL is de-facto most popular implementation of Common Lisp, if I dare say. It is free, it is well maintained, it is easily available, and it is fast. As a student/enthusiast, it doesn't get much better.

Lispworks/Allegro is better if specialized requirements or actual money is on the table. Lispworks supports mobile platforms IIRC which SBCL does not, for ex. But if all one needs is server side deployments or personal projects, there is no need, nor any use of commercial implementations, as good as they might be.