I'm ignoring cheap icecream which is undeserving of the name.
My least favourite quality icecream is chocolate. The whole point of chocolate is that it melts on your tongue. Chocolate icecream is too cold and so pointless, it just becomes grainy so is bad chocolate and bad icecream.
Best flavours are the italian nut gelatos: hazelnut, pistachio etc. I also love fruity icecreams of all kinds (but not sorbet unless it's a lemon top in Redcar).