I don't favor Florence only because the lodging is too expensive and not that great.
Two good days in Florence is plenty, then save money and enjoy yourself more by getting a place in one of the Tuscan towns you visit.
I would spend an extra day in Venice because it's an interesting combination of a lot to do but also very relaxing in the evening after the tourists leave.
My take on Pisa is the area around the leaning tower is spectacular once you get past all the souvenir shops. It's good for an hour or two depending on if you plan to climb the tower.
We didn't think Lucca was great. The wall was interesting but looked more modern than what I was expecting. The rest of the town is flat and plain, not really a hill town. I was there in September and it was lifeless. Other people seem to have really enjoyed it as a less crowded place to see in the summer. It's possible they also liked not having to walk up and down hills like other towns.
Siena is really nice, a night in Siena might be a good option.
Verona was a great surprise for us. It was one of our favorites (maybe because we weren't expecting much). The medieval center and Roman wall and ampitheater were worth seeing. We stayed at the Villa Francescetti Hostel and wouldn't recommend it. Everyone was nice and helpful but it's run down and the water pressure wasn't great.