I like the small Dritz seam rippers. I get them at Walmart for about $2 each. I like them because the tips are small and sharp. Some seam rippers I have tried have a tip that is too big to slip easily under machine stitches.
Here is a link to the one I like:
http://www.amazon.com/Dritz-608-Seam...dp/B005574EBO/
It is quite small, but I find that it is easy for me to handle because it is round. I have not liked any of the seam rippers I have tried over the years that have a flat handle.