I really liked the Riverwalk along the Milwaukee River, I thought it was an uncommon urban attraction. San Antonio's river walk is more well known and I've walked along other riverfronts, but, Milwaukee's felt more urbane because it was closely bordered by buildings and narrow, relatively speaking. I haven't visited San Antonio's which looks to be similar and more colorful. Chicago is due to complete a third and final phase of its expanded Chicago Riverwalk this year, but, the Chicago River is wider with streets bordering the river for stretches. Boston's Esplanade is splendid, although not in the core of downtown, of course.
Cities such as Atlanta may have enjoyable downtowns but lacking a waterfront may put them a notch below in ranking to other cities in my book. Waterfronts attract people. Boston has two waterfronts, as does Milwaukee!