My personal recommendation is Visual Basic, assuming you use Excel for anything beyond recording what you ate for breakfast. VB extends the functionality of it ten fold, if you know a few basic things. It has the added bonus of being a very easy language to learn, the syntax is pretty much English. That being said, no company is ever going to use VB as a real programming language, but it sounds like employment is not your goal.

Edit: Also, it's important to note that (at least I don't think) any language is going to teach rationality any better than any other. It's not like programming changes very much, for most purposes, it's just different syntax.