Quick answer
Te quiero
the everyday version
Te amo
the alternative — see below for when
Which one to use
Te quiero is the everyday one: partners, family, close friends. Te amo is heavier — romantic, serious, the register of songs and weddings. In Spain, te amo between a couple sounds intense; in Mexico it is more ordinary.
All the versions
Te quiero.I love you. — the everyday version.
Te quiero mucho.I love you a lot.
Te amo.I love you. — deeper, more romantic.
Te adoro.I adore you.
Os quiero.I love you all.
Where you are matters
Spain: te quiero for almost everything. Mexico and much of Latin America: te amo is used more freely between partners.
Check yourself
You are telling a close friend you love them. You say: