Formal:
* Prazer em vê-lo(a) novamente. (Pleasure to see you again.)
* É um prazer revê-lo(a). (It's a pleasure to see you again.)
Informal:
* Que bom te ver de novo! (It's good to see you again!)
* Legal te ver de novo! (Cool to see you again!)
* Bom te reencontrar! (Good to see you again!)
Note:
* "lo(a)" is used for a male/female person, respectively.
* "te" is the informal form of "you".
You can choose the best option based on your relationship with the person.