Vitamix would be the golden standard, but a Cuisinart or KitchenAid do pretty well too. However, I had a Black and Decker that was made to do smoothies that did a wonderful job. It was around $25-$30 at Target and it worked as well as the Cuisinart we have now. I gave it to my in-laws when we moved, should have kept that one!
Never thought to try the immersion for smoothies...do you have to thaw the fruit a little or can you use it frozen? Mine came with a blending jar so it would be easy to do...just never occured to try.