If you're asking this question, you probably want the language you're most familiar with.
However if you're picking a language to learn and you really want to do serious scraping, don't pick one that will limit you in the future. You'll want a language that supports threading. While you could debate this, php and python are kinda out. Ruby might be a good way to go. Java could be a good way to go. What about using java and crawler4j? You'll never out grow that.