I have tried several different types, and so far still prefer the plain old small Dritz seam ripper:
http://www.amazon.com/Dritz-608-Seam...dp/B005574EBO/
I have multiples of these, as they cost all of about $2 at Walmart. My latest is neon green (to help me find it quickly!). These fit in my hand well and the small tip is easy to insert into stitches. I have found some of the bigger ones to feel awkward and have also found that some of the bigger tips are not particularly sharp.