The craziest thing I've ever done is inf/ to + inf/gerund

The craziest thing I've ever done is to run/run/running a marathon. Which one is the correct alternative, ESPECIALLY in written English, and why?

I've asked around but everyone has given me a different answer! Some people say you're supposed to use the -ing form or the to + inf, because the bare inf is neither a noun nor a verb. Others have said I'm supposed to use the bare inf because the -ing form would apply to a habit, something I do regularly as opposed to only once in my life! Others have said you can only use the -ing form if it's at the beginning of a sentence (like "Smoking is bad").
As you can see, it is SUCH a divisive topic!