I've lived in most of the states at one point in my life. This question is hard because there are advantages to many states, and even general locations. I'd say, usually, it's best to be near one of the largest most populous states, but not in it. On the east coast, being near New York is great, but not in it. The cost of living there is high etc. Vermont (I lived in Putney Vermont and a few other places there when young) is probably a good choice. You'r fairly close to NY and can easily get access to the great foods etc. Low crime rate, low cost of living (Very low in some places), but it can be a bit cold (not a factor for me, but could be to some).
On the west coast I'd say Washington is probably one of the better choices, especially near the boarder to Oregon. I lived in Vancouver WA for a year and it was great. You could jump over to Oregon to buy things without sales tax, and you have no state tax either (not a great place to live if your on minimum wage though!!). It's not THAT far to drive down to CA if you want and there is water all around (rivers, waterfalls etc).
Sorry, but i'm knocking Texas, Arizona and Florida all out of the running. I've lived in all 3 of those states (or served there in the military) and they all have serious problems. In Texas you can go from one extreme in weather to another in one day. We had days that would start out freezing and end as a black flag day (over 90 degrees). Arizona is just plain freaking hot. It's expanding quickly, and air conditioning can help, but it's freaking too hot (i'm more concerned about heat then cold as I can handle 20 degree temps in jeans/tshirt...but 90+ temps ruin my day). I was born in florida and lived there awhile, but lots of insects, muggy/hot conditions, extreme weather (Which I generally like but not many do) and high cost of living all make it hard to recomened. For some reason people like to retire there though.
If you have to pick one of the highest population states, I'd go with California. It's cost of living is sky high, gas especially, but there are still places in CA (away from the bay area/LA etc) where you can live fairly close to the cost of other states. This gives you the advantage of still being IN CA and close to places like SF and LA etc, so if you do want to travel there, and visit the coast etc, you can do so fairly easily/cheaply. My wife for instance lived in Apple Valley (I would not recommend it) when I met her. Way to hot (reminiscent of Arizona), but she was renting a 2 story home for $500 a month (would be more now, but not much more). Meanwhile, where we live now (the Bay area), it would cost $3000+ a month for the same thing.
The best states to live in depend a ton on your own circumstances. How much money do you have/what is your job, what kind of weather do you like, do sports or other activities matter to you, do you like to live in large cities or more rural areas, do you need/want to be on/near the water etc. To me, anywhere exceedingly hot is instantly crossed of the list (even with air conditioning). I personally LOVE rain (one reason I like Washington so much), and I prefer Rural areas to large cities. I like green, I like water, I like clean air, and I like room to breathe. I'm not overly concerned about the cost of things (many of the middle states are far cheaper for instance, so if cost is a large factor, they may be better for you), and most of my hobbies can be done anywhere or are online. Vermont for instance can be a bit isolated in some areas, but that is not a factor for me at all.
I would venture to say that most states have great places to live. Even a few places in Arizona where kinda nice (Except for the weather..which is pretty much horrid everywhere in Az). Florida I'm sure has some places that are not bad, and Texas probably has great cities as well (maybe the crazy weather was just while I was there and not common). I certainly know that in CA (where I have lived the most) there is a huge difference from one location to another within the same state. I still have never found that perfect place for me, and I'm getting up there in age. Pretty soon location won't matter much as I won't go outside except to shop/eat out or go to the movies. Maybe I'll eventually figure out why everyone retires to Florida and move there as well!!