When looking for race/class combinations, especially for pregens, the main 4e theme I have is: make sure the racial stats match up with at least one primary or secondary.
Given that each character will be built with point-buy, they should all end up with +4 or +5 in their primary stat, depending on race.
Have 3-4 deadly-dull stereotypes for the munchkins (literally, young power gamers) in the crowd. I know I certainly wasn't aware of stereotypes at that stage of my life.
In points of light, avoid using monster races for a one-shot, it's too easy to have group-conflicts of "you're not acting right."
So, with all of that said, looking at this purely from a "what's the most interesting build?" We turn to our friend the charOp boards and I'll just suggest interesting combos for each class.
For a con game, just take all the light blue first level options for the respective build. It makes it less likely to have a dud character. Give everyone foo expertise for free because hitting more often is fun (and it is a math fix. Keep an eye on the monsters you're using and try to favour the MM3, but that's a topic for another question)