I would also go with Hyatt Diamond. Marriott can offer you about the same level of hotels as Hilton, so you're covered in that category. Hyatt always seems to have a ton of offers for bonus points, so you can build them up pretty quickly. Likewise, some of the high-end properties are really nice. Not that Hilton has poor properties, but they are usually in locations that the prices are so cheap that it doesn't make sense use points.

Mike