cho'gath, diana, lux, orianna, ap nid, vladimir, akali.
essentially, any champion that can either soak damage on predictable incoming damage (diana, lux, orianna) or do well vs harass champions due to natural sustain (cho, vladimir, nid) can play extremely well vs Kassadin. Kassadin's strength is the same as LeBlanc's. he gets in and does damage while the silence is running making every single trade unanswered, much like LB's W > Q > R > E combo. if the champion you're facing can soak your harass/retaliate after your silence wears off you're going to have a bad time
also if Ahri tosses out the charm at the same time as Kassadin null spheres you're bound to take damage.