I suppose it depends on how different the second language is from your native language. As in, Dutch may not offer a big boost in new ways of framing the world for a native German speaker, for instance, since they're closely related languages. (This depends on what you mean when you say "cognitive benefits"; I'm assuming here some form of the Sapir-Whorf hypothesis.) In my case, I have found English especially adaptable (when compared to my native language) when it came to new words (introduced, for example, for reasons of technological advancement -- see, for example, every term that relates to computers and programming), since it has very simple inflexions and a verb structure that allows the formation of new, "natural-sounding" phrasal verbs. Having taught my own language to an American through English, I wouldn't say the same about it expanding your way of conceptualising the world, unless you're really fond of numerous and often nonsensical inflexions. I'm not sure I could recommend specific languages that may help in this regard, but I think I could recommend you to study linguistics instead of one specific language, and use that knowledge to help you decide in which one you want to invest your time. I've studied little of it, but the discipline seems full of instances where you put the spotlight, so to speak, on specific differences between languages and the way they affect cognition.

I would expect they have the correlation backwards. Smart people are more likely to find it easy and interesting to learn extra languages.