If we are talking about best places to live, I would say the best are:
1. Highgarden - Green fields and happy people. What's not to like?
2. Sunspear - It's warm and isolated from the bullshit in the rest of Westeros.
3. The Vale - Sure, you can't use the Eyrie in the winter, but it's so badass.
If we are talking about the best castles to be the lord of, then I would say:
1. Casterly Rock - The most money, therefore the most power.
2. Winterfell - You pretty much run things in a very large area. Sure, winter sucks balls, but if you are the lord, you will be taken care of.
3. Highgarden - Sometimes food is more powerful than gold.
I think the worst places would be:
1. The Iron Islands - You have land that cannot be worked and lead a culture of people who pillage for a living and drown themselves.
2. Storm's End - It's a small, rocky province. It sounds pretty boring.
3. Riverrun - It seems like the lord of Riverrun has plenty of power, but it also seems to be the center of most of the fighting, leading to burned fields and hungry people.