Formal:
* Dobranoc, moja miłość. (pronounced: doh-brah-notch, moi-yah mee-woshch) - This is a very formal way of saying it, appropriate for someone you're not very close to.
Informal:
* Dobranoc, kochanie. (pronounced: doh-brah-notch, koh-hah-nee-eh) - This is a more affectionate way to say it, suitable for someone you're in a relationship with.
* Dobranoc, skarbie. (pronounced: doh-brah-notch, skar-bee-eh) - This is an even more affectionate way to say it, often used for someone you're very close to.
* Dobranoc, moje życie. (pronounced: doh-brah-notch, moi-eh jee-tsee-eh) - This literally translates to "good night, my life," and is a very romantic and passionate way to say it.
You can also add a "kocham Cię" (pronounced: koh-hahm tsee-eh) at the end, which means "I love you," to make it even more romantic.
For example:
* Dobranoc, kochanie. Kocham Cię.
* Dobranoc, moje życie. Kocham Cię.
The best way to choose which phrase to use will depend on your relationship with the person you are talking to.