Very Informally: 待ってるよ ・まってるよ Informally : 待っているよ・まっているよ Formal: 待っています・まっています To a close friend I would say one of the upper two, but to someone you don't know too well/is your senior in some regard (unless you've established that you'll talk informally with each), the last one is better. I assumed that you want to say it to someone because they're running late, or they're asking you if you'll wait for them, so that's why I chose the above ways of saying it. If it's not one of those cases, please let me know!

あなたを待っています。 bonus: 私はいつでもあなたを待っています。 I am always waiting for you.

私はあなたを待っている / います。 Watashi wa anata wo matte iru / imasu.