I use the cheap $30 rapala ones from Walmart and have never had problems with them. I'm on my fourth knife. They've lasted for about 1,000 crappie each.
I went to the manual Bubba Blade last fall and have gotten almost same speed with it as the electric blades. I went with the 7" taper flex, but probably going to get a 9" taper because the ribs bones do give a little bit of trouble on the larger fish.