Formal:
* Estou feliz em ver: This is the most literal and formal translation.
* Fico contente em ver: This is a more polite and common expression.
Informal:
* Estou feliz de te ver: (with "te" for "you" in informal situations)
* É bom te ver: (Literally "It's good to see you")
* Que bom te ver!: (Literally "How good to see you!")
Specific Situations:
* Estou feliz em te conhecer: (I am happy to meet you)
* Estou feliz em te reencontrar: (I am happy to see you again)
The best way to say "I am happy to see" depends on the level of formality, your relationship with the person, and the situation.