All of them should return from time to time, but at the moment we need less of toon Link and adult Link as they have been seen plenty in recent games and we need more kid Link. I haven't seen a good kid Link since MM and I started off playing with Zelda with kid Links so I miss what his innocent yet serious epicness that he came with him. Adolescent Link has not made an appearance in a while, but I'm not really missing him as much as kid Link. I guess its because the other Links all together fill the void he leaves. I'm putting kid Link as my answer, but this is something that changes as time goes on.
The thrill of being a 10 year old kid and defeating someone as powerful as Ganondorf is amazing. However, being an Adult is cool because you can show off your strentgh and dispose of him easily. Its very close, but im going to say Adult Link.
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.