My number one choice would be Westminster - PRIME residential, LOW council tax - you'll never have a problem letting it or re-selling if you should wish to do so. Bloomsbury is a great area (I live close by) but DEAD at the weekend or swarming with tourists in the summer - if you like that, fine, might drive you a bit potty. But the Brunswick is very handy, great shops and restaurants and a lovely Waitrose.
My next choice would be the Borough flat. Only because I prefer the other areas, and Southwark council tax is a bit higher than Westminster and Camden (when I last looked). On paper, this could be the best investment. It's no coincidence that the Duke of Westminster has sold up a lot of his prime Mayfair properties to buy in Borough and its environs because that area is set to boom. Personally I don't love that area. Swiss Cottage is a bit yuk if you ask me, I have no idea why people pay through the nose to live there, (Finchley Rd is grim if you ask me) but as Regent's Park is wonderful being near it is a plus - but you would be near it in Bloomsbury anyway. Kennington is ok, but but not as great as the other areas. Having lived in all of the areas you mention (bar Borough), that is my opinion but it's a personal one - they're all great, convenient and safe. Hope you find the place you want OP.