If I could only have one lens, and cost wasn't an issue, I would chose the 24-105L. Second to that if I really wanted the zoom, I would get the "EF 28-135mm f/3.5-5.6 IS USM", it sells new for about $420, I'm sure it would be a lot less used if you can find one.
If cost is a major issue I'd take the 40mm f/2.8. In fact I often just walk around with that lens since it's so nice and light. The 50mm f/1.8 is also a good option.
Also if you sell the 580ex, you should be able to pick up a knock off flash for around $80.
To drop the 5D2 for something else would be a hard thing to do. However, if it needs to be done, you can get older used rebels with kit lenses (like a t2i/t3i) for very cheap now days - and they can still take great pictures.