There are relatively few to choose from in Winchester itself. St Swithuns or Princes Mead. Slightly further out you have Twyford. Where are you living and what sort of school are you looking for for your dd?
I have no personal experience of either but know people with children at both, how old is your dd? How academic is she? I only ask as I have heard of a few examples of girls being effectively pushed out st Swithuns junior if the staff are unconvinced they will make the grade for the senior school.
I cannot recommend Prince's Mead enough, both my daughter's have attended this wonderful school and both have thrived. It is a warm, friendly and caring school with a family atmosphere where the teachers are not only excellent but also genuinely care for the children. The parents are engaged and involved. Both of my daughter's went from being extremely shy to confident in themselves thanks to the school environment. All children are given a chance to participate and shine and we have been thoroughly delighted with the school at all times (my eldest has now moved on, but my youngest is still at the school and extremely happy). Hope that helps!
A friends daughter went to St Swithuns and it was lovely.
However I think as above Winchester is an odd place in that Is be happy with my children at any of the comprehensives. I'm not overly sure that private would be worth it. Winchester is quite a different area to other areas I've lived. I'd love back in a flash!
If you have an alpha type child then they can do very well at Twyford. However if your child isn't in about the top 10 for any activity in their year then they may not get to do all the things that you/they want. This includes sports day where many children only get to take part in the tug of war at the end of the day! They value sporting achievement very highly, which goes one of two ways for the children. They have Saturday school from year 4 and you should check how long the school day is as it gets pretty late as they progress.
Prince's Mead prides itself on being a nurturing school, however it is smaller than Twyford and St. Swithuns, and some people feel it doesn't have the "badge" of Twyford. This I think is more an issue for the parents than the children. It only goes to year 6, no Saturday school. Pastoral care is very good.
St Swithuns has a good reputation at senior level, not so much at junior. It used to be the case that if your child went to the junior school then it was a bit of shoe in that they'd be accepted to the senior school. This is definitely no longer the case.
The state schools in Winchester are generally good, but you've still got about 30 kids in a class and the opportunities for sport/music/art are just not on a level with the independent schools I'm afraid.