Scream was GREAT, still very effective after seeing it 100 times.
Scream 2 was great up until the "final reveal" which was sort of cop out but still highly entertaining. Loved the opening scene.
Scream 3 was totally rushed into production, and you could tell. The whole feel of the previous two movies were completely lost. It had its moments, but it wasn't as funny or as clever. They should have never changed writers. It was disappointing.

Huge Scream fan here though, really revived the horror genre.