To me, I would want the experience to be both practical (see if I could gain powers or gain something of value) and enjoyable (a comic book person I might want to actually hang out with).
I was initially leaning towards Galactus or the Oa Guardians, simply to gain power cosmic or become a GL (or even just have access to their GL super computer with a ring that has no charge. I could make a monetary killing with that information) but both sets would probably just ignore me the entire day and I would not get much utility out of them unless I got powers.
Not Deadpool. Seriously, the guy is hilarious, but put yourself in the situations he tends to find himself in and you end up dead. And he wouldn't really care either way.
If I wanted to simply learn more about the world, Black Panther would be my pick. Not only on how to rule a nation, but for the cultural experience.
The tech guys appeal to me, but Tony, Reed, and Mr. Terrific would never make tech for a guy they met in one day unless I did a convincing sales pitch on an invention I've thought of before which they could obviously make and design better than I ever could. Which could work out well with me if this day thing turned into a business partnership. Box might make me tech if I played my cards right and would probably be my first choice being that he could make it the fastest since he could whip it up right then and there exactly to whatever specifications I want.
I don't really know who I would want to pick honestly. There are tons of comic characters I would love to associate with, be it for their knowledge, skills, value or personality.
Blair Wind makes a good point, you don't want to hang with those who already get an insane amount of publicity. Reed Richards and his kind are all so responsible. They do a mass of charity and such, why would they hand some guy they barely know advanced technology? Go with someone a bit less known, someone who wants to make an impact or be remembered and your chances increase drastically.
Also, having a couple of beers with Guy sounds awesome.