Common Lisp.<p>It has standarized, well-documented, proven-for-decades way of doing necessary, common day to day stuff like conditions and restarts (aka "exception handling"), package system, or object-oriented-programming. Consider, for example, that Scheme has no standard way of organizing your code into packages and namespaces. Nor standard exception handling system; you are expected to use a lib, use <i>what the particular Scheme implementation you are using offers</i> or roll your own using the (extremely powerful) continuations feature. Scheme also doesn't have a standard OOP system like CLOS. You can implement all of this in Scheme, but Common Lisp has this standarized, proven, well-documented way for all of that. This makes reading others' people code easily, because common stuff is going to be done in a standard way that you already know.<p>It is a Lisp-2 which, for me, makes programming comfortable. People always talk about how macros in Common Lisp are "unhygienic" by default, but it is trivially easy to write a hygienic macro on Common Lisp.<p>It is an ANSI standard and the Common Lisp implementations largely comply with the standard, which means that I can take my code and run it with no changes (or very slight changes) on awesome Lisp implementations like LispWorks, SBCL, CLISP and many others.<p>There is a big amount of documentation available and in the last 10 years the amount of libraries, books and tooling has increased to make CL programming nicer than ever.<p>The implementations can be really high performance. SBCL can be had for free and it's performance is awesome. It is amazing that a dynamic programming language could be <i>that</i> fast. Also the implementations are mostly very nice to the programmer.<p>Common Lisp also allows to do low-level stuff if you like, for example it has full support for bitwise binary manipulation. Numeric support is magnificent and standarized: All CL implementations support real, fractional, complex, int, arbitrary precision numbers, and work with them really quickly.<p>Clojure seems more limited compared to Scheme or Common Lisp, being tailored for doing everything the functional way; while both CL and Scheme allow you to be 'eclectic' and use whatever programming paradigm the situation calls for.