to counter fiddle is all about disabling his use of crowstorm. if he cant use it, he cant do much damage to a team. only drain a single target.
so the best counters are those that manipulate where he is. basically anything with a "push" effect, so as mentioned above, alistars headbutt, janna's ult, vayne's condemn, gargas's ult, and theres probably many more
also stuns do work, but not in the way your thinking of it, you can stun the fiddle and that'll stop him from moving, everyone in the radius should already be trying to put distance from themselves and the fiddle so the stun will help get you out of there without him following.
and last, everyone should have some form of MR against a fiddle, not getting it is just suicide, theres tons of types of MR items and theres bound to be 1 to fit in your build, if you think that your build has no room to fit a MR item, then your clearly doing it wrong. there is not 1 champion that doesnt have room for flexibility and thats the only way any champion should be played