I'm sure you'll get lots of recommendations but for what it's worth I was at The Double Tree by Hilton in January and ideal for 3 nights. Great hotel, nice room with a big balcony, spotlessly clean but the breakfast (included in the price) was average but there is a big supermarket just round the corner if you fancy using that. The hotel as you may see is at one end of South Beach and so quiet and not noisy around the area.

Beach shuttle bus across the road, safe area, friendly staff, a few minutes walk to the beach/promenade and very walkable to the Art Deco area as well as the City Tour bus stop across the road from the hotel (turn left and cross the road 100 yards along)

I saw an awful lot on that bus tour, hop on/hop off bus and would highly recommend it.

The Gates Hotel South Beach - A DoubleTree by Hilton

Pete