I am not sure which one is the "best", but I can recommend some widely used datasets in my research field.

1. Oxford Buildings

usually used together with Flickr 100K and/or Flickr 1M datasets

2. INRIA Holiday

small-scale but widely used

3. Kentucky Image Retrieval Dataset

a bit old, small-scale, and easier than others

4. TRECVID Instance Search Dataset

an image-video retrieval dataset; very large-scale; very challenging; updated almost every year

5. Flickr Logos 32

not very widely used but fairly challenging and interesting to me

Also i recommend the dataset used by the mpeg-7 standard called "common color dataset (CCD)"