Formal:
* Prazer em conhecê-lo/conhecê-la. (Pleasure to meet you.) - This is the most common and polite way to say it.
* É um prazer conhecê-lo/conhecê-la. (It's a pleasure to meet you.) - This is a slightly more formal way to say it.
Informal:
* Prazer em te conhecer. (Pleasure to meet you.) - This is the informal way to say it, using "te" instead of "lo/la."
* Que bom te conhecer. (Nice to meet you.) - This is a very informal way to say it.
You can also use "Prazer em conhecê-lo/conhecê-la" followed by the person's name:
* Prazer em conhecê-lo, João. (Pleasure to meet you, João.)
Remember to use the correct gender pronoun for the person you are speaking to:
* lo for a male
* la for a female
* te for both male and female in the informal context
It's always a good idea to be polite and use the formal version unless you know the person well.