what is the best way to write hope in one symbol in kanji?

People will not like this, but your best bet is to not get this particular tattoo. Unless you PERSONALLY know someone who is fluent in either Mandarin Chinese or Japanese, you cannot guarantee that the character you are getting tattoo'd on your body actually means what you think it means.

Case in point, a friend of mine that got "free spirit" in kanji that actually said "open fart". True story. Large cover-up tattoo followed.

I would suggest getting an original peice that represented hope to you. Most tattoo artists are more than willing to work with you to get the tattoo you want. This I know from lots of personal experience.

Hope doesn't exist in any way as one symbol

Kibou 希望.

However, you could use love.

ai 愛

Hope, Nozomi, or bô, is 望.