Isabelle is my favorite, and it's the one I've used the most, if only because sledgehammer is super cool, but Coq is neat as well. It has coqhammer (a word I will not say at work), and that works ok but it didn't seem to work as well as sledgehammer did for me.

Coq is much more type-theory focused, which I do think lends it a bit better to a regular programming language. Isabelle feels a lot more predicate-logic and set-theory-ish to me, which is useful in its own right but I don't feel maps quite as cleanly to code (though the Isabelle code exporter has generally worked fine).