I would say
1. KTM 250 (if you can find one)
2. RM250B
3. YZ250D
You want a good overall (power and handling) bike that is dependable and has a good parts network. I have a 1978 Husky 250CR and it is a great bike, but if you read the articles on it you will find out what most magazines felt about the '77.
Unless you have an emotional attachment to a bike from the past (had one or always wanted one) why beat your head against the wall. A Vintage MX bike should be fun not something that sucks up your time and money and then ends up on eBay for a lot less than you put into it. (wait a minute, don't they all?)
I've learned that lesson a few times and when I was looking for Pre-78 big-bore I decided I wanted a '77 Husky 390 (first year motor, first year frame) until I looked at one of my old Cycle World mags, that tested both the YZ400D and the '77 390CR. The 390 was built for a single purpose; To give someone who could ride it as hard as it needed to be ridden a great chance to win! The YZ400D on the other hand was built to be very adaptable to almost every rider. It is easier to ride and easier to get parts for. After finally racing it I'm glad I made the decision to go with the YZ. The first 390CR would be a cool bike to have, but it is not for everyone. So, my '77 390CR motor sits on the shelf waiting for the day I may decide to restore one.