MAC's 217 is my favorite, MAC also came out with a new one with the Raquel Welch collection, the 223 Eye Blending Brush, which is more precise and in my opinion a good investment, though many people can't tell the difference between the two. Personally I think it is more precise and picks up color easier
217 and 224 are the best blending brushes. I have the LE 223 and it's ok ... nothing special. If you can get to a MAC store the 222 is also amazing. BUT your basic blending brush is the 224 ... I love the 217 because I feel like it picks up more color but I ALWAYS use both on myself.
I'd say go with the 224 I just adore mine. If you want a cheaper version of it you can go with the Sonia Kaushuk stuff at Target. She's actually got some great stuff at very reasonable prices.