The best time to visit Ahmedabad (anywhere in Gujarat, actually) is during Uttarayan or Makara Sankranti, in my humble opinion. That is when the citizens go all out to enjoy themselves, when the kite mania is on, when you get to see the city in a whole new avatar. This is apart from the Navratri season, when the city is decked up at its glorious best, of course.
Read my post about Uttarayan and the kite fever in Ahmedabad, just in on my photo blog!