Japanese equivalent of crazy?

I am looking for this word since a long time...

I learnt in my dictionary that an equivalent of "crazy" is "kichigai". So I used it once with a good Jp friend (I was not talking about him). He laughed and told me that this word was extremely vulgar. So it was ok, but I'm afraid to shock some Jp person in the future by using it.

I wanted to know what Jp word would be a more polite equivalent of Crazy.

Just imagine that, for example, I want to say "I met a crazy man in the street, he was talking alone"

It seems like Kichigai is more close to "mentally deficient".. that is too agressive.

Thanks for your replies!

a candidate is 変な, not normal, not ordinary, strange, queer.
通りで変な男に会いました。独り言を言っていました。
the word きちがい is too definitive and strong, no question to your judgement of kichigai(mental illness).
the word 変な is more moderate, it implies that you think he is crazy, but some other persons may not think so.

The word "kichi x ai" is so banned today that they even changed the title of the classic movie "Kichigai Piero" (Pierrot le Fou) into a word that doesn't really exist which is "Kigurui Piero."

Anyway, in the context of your example, I would use "atama ga okashii hito" or simply "hen-na ojisan" but note that that doesn't apply for all things "crazy."