i'm assuming you're coming from a pre-cleared canadian airport into the US. In that case, I would vote ORD. I've transited domestically or domestic to intl dozens of times and have only had one connecting flight significantly delayed (which helped since my arriving flight was also significantly delayed).

The one kind of transfer I don't like making at ORD is intl to domestic and dealing with immigration which is all concentrated in T5. Actually, I don't think there's any airport where I enjoy the going through immigration mess...it's just the thought having to re-clear security, standing in line at the passport desk, then get my checked bags to carry them 200 ft to the re-check line, blech!

actually thinking about it, intl travel outbound from the US is a whole lot more fun than inbound. Outbound, you only clear security once (most of the time), your bags are checked all the way through to your final stop, you get to utilize the fancy departure lounges (assuming you have access), it's all great.
Inbound, it's nowhere near as nice. Some airlines don't even let you visit lounges on inbound domestic connecting flights, and having to deal with immigration in the middle of the trip, claiming my checked bags twice, going through security twice...