Favorite world-wide city I think would be an easier answer for me, to be honest - Barcelona, hands-down Barcelona!! Best trip I ever took! But trying to pick my favorite city in the US proves to be a harder choice...thing is while I love to travel, I also love to be home! I love the city I live in and I love to visit the city I grew up in. I have had some great trips in the US, though. Coast to coast I've had a great time in both NY & LA - closer to home I San Antonio & Austin (many trips there) - college towns are soooo much fun, though those "fun days" are in the past! Hee hee!
So, to choose now I think I would have to say that I love Houston - it's home now even though I didn't grow up here. We have so much in Houston - arts, sports, NASA, little-bit-of-country & little-bit-of-city...I can't say I like the weather & the traffic, but if you're a Houstonian that's something you'll learn to live with!
For my 2nd choice I think I would have to pick a place I've never been, but WANT to visit instead...I have never been to Washington D.C. and I would really love to go! I would love to go when the Cherry Blossoms are blooming - I've seen pictures and it looks so beautiful. I would love to see our national monuments.
Before I got married I used to travel a lot - now that my kids are little we stay close to home, but I truly hope as they grow we'll continue to show them all the great places that are around us.