[Tangram]

'Suki' is a versatile term. You can put 'na' on the end to say something is your favorite, but used in context here, it is definitely what you would say to a girlfriend or boyfriend. If you said suki in reference to any other person, it would come off as a little...weird.

Moving from 'suki' to 'daisuki' is the equivalent of moving from 'like' to 'love' in English. So in a serious, long-term relationship, you would use 'daisuki'.

The use of 'aishiteru' really comes down to personal preference, but you would definitely never, ever say it to anyone that you weren't already engaged to (or otherwise planning to marry). Even then, some people tend to avoid it. You hear it mostly in movies and anime where the characters' romance is based on the idea of the red string of fate or something similar. I'm to understand that most couples only use it comfortably after going through some sort of crisis together, even when they're married.

For instance, a married couple tries for five years to have a baby. The wife gets pregnant, and loses the baby at 7 months. For a while afterward, their marriage is very rocky. The wife confronts her husband about fearing that their marriage will end, and he tells her not to worry and says 'aishiteru'. And even though he's said it that time, that doesn't mean they'll completely transition to 'aishiteru'. They'll still probably use 'daisuki' for the most part.