Before answering, I'll give the following personal definitions for each:
Adult Link: Link at age 17 or so, as seen in the adult part of Ocarina of Time, Twilight Princess, and Skyward Sword. (I'd say he's about 17 in all three of these)
Toon Link: Young Link, with the whole cel-shaded look as seen in The Wind Waker, Phantom Hourglass, and Spirit Tracks.
Kid Link: 10-12 year old Link, as seen in the first part of Ocarina of Time and Majora's Mask.
Adolescent Link: 13-15 year old Link, like in the original and A Link to the Past.
I have to go with Adult Link. My favorite-est Zeldas all have the adult Link in them, and I don't think it's a coincedence. Adult Link tends to be surrounded with a gritty, mature, epic aura, and, as such, is in games with that feel. He's just my favorite age of Link. He's the most heroic and generally awesome.