Many years ago I lived in the Swiss Cottage/Frognal/Belsize Park neighborhood and it was nice, but not any nicer than other neighborhoods. Now, however, the prices have shot up considerably and its still just fine, more or less. St Johns Wood has always been pricy, but its a nice, nice area (heavily Jewish though - not sure if that matters to you or not, but there are a lot of temples and Jewish-focused schools and the like).
We are moving shortly to a place in West Hampstead and that area has changed quite a bit in 15 years. Much nicer - the West End Lane is a cute little area tucked back behind some very imposing (and expensive) mansion blocks. Also, that area is extremely well connected either via Overground to get to the East End, by bus/Tube to get into central London/West London, or Thameslink to get into the City quite quickly. Finally, they built a giant, hideous mall structure by Finchley Rd tube around 2000 that does have its uses for a really large Sainsburys, Waterstones, movie theatre, etc.
Maida Vale and Little Venice are also very pricey - the closer you get to Regent's Park or, god help you, Primrose Hill, the higher the prices go and the more famous your neighbors.
I don't know Kilburn or Queens Park very well - I think if you are on the border with some of the areas above you will be ok, but I have heard that parts of Kilburn can be questionable. There is also South Hampstead to check out too, though that is pretty stylistically similar to West Hampstead