If it's in a romantic way, it's almost always going to be "te quiero" (why would you use the formal with someone you're in love with
), BUT, some areas use exclusively (or almost exclusively) the usted form even for kids and good friends/family. This was the case when I was in Costa Rica. If you want to say I love you (Maybe to a grandma or aunt/uncle) and use the usted form, it would be "Lo quiero" or "La quiero" depending on if it's a man or a woman.
As a side note, the initials TQM are pretty common at the end of letters/emails etc. It means Te Quiero Mucho.