Well for me personally it would be

1. La Crosse, WI
2. Cardiff, Wales, UK
3. Des Moines, IA
4. Washington D.C.
5. Minneapolis, MN

Bet between your choices, I really do like DC, I've only been there twice for day trips though. Chicago is great with all the museums and parks and things, but I live so close to there I often forget about how fun it can be.