Declarations of love

3

Reserved for solemn moments: start of relationship, anniversary, proposal.

我爱你 wǒ ài nǐ I love you (strong declaration, use sparingly)
我爱上你了 wǒ ài shàng nǐ le I've fallen in love with you (上...了 marks the change of state)
你是我的一切 nǐ shì wǒ de yīqiè you are everything to me (very romantic)

Everyday "I like you"

3

Lighter and more common than 我爱你 in Chinese couples.

我喜欢你 wǒ xǐhuan nǐ I like you (the ordinary declaration at the start of a relationship)
我喜欢上你了 wǒ xǐhuan shàng nǐ le I've grown fond of you (evolved into attachment)
我很喜欢你 wǒ hěn xǐhuan nǐ I really like you a lot (很 reinforces)

Missing someone

3

Often more used than pure declarations of love.

我想你 wǒ xiǎng nǐ I miss you (想 = to think of, to miss)
我很想你 wǒ hěn xiǎng nǐ I miss you a lot (很 reinforces)
想死你了 xiǎng sǐ nǐ le I miss you like crazy (emotional hyperbole)