Any function can make me happy or not depending on how its used. I guess I'd rank them as follows, with happiest first.
Ne: If I had to pick one function that makes me the most happy overall, this would be it. Ne allows me to make interesting connections between things and thinks of new possibilities. Much of my sense of humor and childlike enthusiasm stems from this function.
Se: I don't really use it that much and its one of my weakest functions but when I allow myself to fully live in the moment and completely immerse myself in my senses, I think to myself, "why don't I do this more often?" I think I feel kind of guilty about just indulging in Se. I usually let other functions override it. I should think more logically. I should think more about the future, etc.
Ti: I really enjoy playing with logical systems in my head and analyzing how it all fits into a whole. I love finding out the why behind things. I guess I rank Ne as making me happier because I think as a Ti dominant, I tend to just take it for granted.
Si: I enjoy this function for the nostalgic effect and reminscing about the past. I enjoy the sensory impressions it sometimes creates. On the downside, my Si can recapture unpleasant past memories and sensations I'd rather just forget.
Te: On the plus side, I feel really good when I've effectively utilized Te to accomplish tasks efficiently and effectively. On the downside, Te seems awfully dry to me at times. It's too mechanical, too procedural.
Ni: I enjoy Ni for the insights it creates and the ability to see multiple perspectives on the same thing. On the other hand, my Ni has a dark side to it, imagining worst case scenarios and I think the source of some of my existential angst comes from here.
Fe: This is a troublesome function for me that causes me undue anxiety at times, especially in the social arena. When in the grip of Fe, I get powerful emotions that are hard to control and hypersensitive about others' feelings towards me. I guess it makes me happy in the sense of having good harmonious relations with others.
Fi: This is probably my most troublesome function aside from Fe and I wouldn't exactly consider it my happy place (is this a common attitude to have towards one's demonic function? For INTPs that is Fi) When I'm really down and depressed about something, that's when my Fi goes into overdrive. On the other hand Fi is good for realizing the times when I should have listened to my heart and to not ignore what I feel deep down. Sometimes Fi is confusing, when values conflict with each other or when doing the 'right thing' seems to conflict with what you personally value.