Hi Morgan and welcome to the forum! :O)
Firstly, have you felt out the willingness of the other 11 people to spend what it will cost to go to Europe for your birthday? It will involve a considerable amount, and possibly some complex arrangements for family with pets, very young children, or elderly members with mobility challenges. if applicable. It's just not clear if your family has shown interest to this idea before launching into recommendations? You are wanting to travel during what's high season in most of Europe, virtually all the places you list will be very busy, and accommodation prices will be at peak so you need to take that into consideration. You also need to take into consideration that your choice of location may not be of interest to them.
LOL, we've been faced with invites to any number of destination weddings we've declined due to the cost and footwork of going to locations we've little interest in. Family and friends have done the same.
Additional consideration; rural Tuscan villas, for instance, are not great locations for a group needing to get around by public transit to those "things to do". For people to be able to come and go independently - I wouldn't plan on everyone doing everything together- you're going to want to be walking distance from public transit or in an urban area where most sightseeing is reachable on foot. Driving in Europe isn't everyone's cuppa tea, isn't recommended for every location, and involves some homework you'd be expecting your family members to do.
And just one more consideration: Is your family expecting you to make ALL the sightseeing arrangements and play tour guide/social director every day of this trip? How well traveled are they, and are they willing to do a fair amount of research and bookings themselves? How 'on call' might you need to be every day?
I do NOT wish to throw cold water on your dream!!1 It's just that there's' more involved than location. There is also no 'best' unless you've had the conversation with your family about what they're willing and able to do. You might want to have the Plan B of planning a wonderful European adventure on your own or with a much smaller group if it all becomes too much to make happen?