Public Policy Polling says Seattle is number one:

The Pacific Northwest has a good reputation nationwide—the two most popular of the 21 prominent cities we asked about in our national poll last weekend are Seattle and Portland, OR. 57% of American voters see Seattle favorably and only 14% unfavorably, edging out Portland (52-12) by three points on the margin.

The most unpopular is Detroit, which only 22% see positively and 49% negatively. Americans have net-negative impressions of only two other of these cities, and both are in California: Oakland (21-39) and Los Angeles (33-40). In February, PPP found California to be the least popular state in the union. It does have the 11th most popular city, though: San Francisco (48-29).

Read the whole likability report right here.