Considering that I haven't been on 13 yet (probably not missing much though), My favourite ride is either Spinball or Nemesis.
Spinball because of it's spinny goodness, nice retheme, a good track layout and decent length.
Nemesis because of it's brilliant track layout, it's closeness to the ground, brilliant theme, it looks good and it has a quicker single rider throughputs than most of the other rides. Short though.
Overall, I think Nemesis is my favourite ride but with Spinball a close second.