I can just give you my experience, I cant say who is for sure the best because I havent tried all of them on a regular basis. I've used Bittrex for a long time and they have always answered and dealt with any queries I had, they seem to operate a tight ship.
Also I found localbitcoins useful which deals with escrow for people changing money and other cash value into and from Bitcoin. Both useful and generally well run with a decent interface for new users.
I dont like when people say invest, exchange yea and BTC is a kind of value exchangeable currency just like EURO if you went on holiday to Europe. It has a use especially in travel or on the internet globally. If talking about putting the BTC into a place that will return a yield, a kind of investment then sure we can discuss that also