I'm not sure why the number of direct flights from ORD matters, as long as there is one to get you to the hub.

We like to fly to AMS from ORD. It puts you within very easy driving/training distance from many parts of Western Europe and a bonus is the new 767 F/C cabin if you're looking beyond Y. The downside is that AMS isn't a big *Alliance hub, so connections aren't that great.

If you're looking to connect somewhere else the only good options are FRA and MUC, both of which are well served from ORD, and have many onward connections on LH.

Avoid LHR at all cost.

If you're willling to extend your ORD-Europe flight to other *Alliance partners many more destinations open up. Copenhagen via SAS, Vienna via Austrian, and Switzerland via Swiss for a few examples.