There are a number of small boutique hotels that are very nice, Art-Deco type hotels. You may want to check out Miami.com as they list many of them.

The top 'mainline' type hotels on SoBe are:
The Loews, Ritz Carlton South Beach and the Marriott is also quite nice. The Fountainbleu has been renovated as is very nice, although it not in the middle of South Beach.

Hope this helps.