The top two exchanges in my opinion, and this is just from my personal experience, are coinbase.com and bitstamp.net. Between these two exchanges, especially if you can make a wire transfer and ACH transfer, or a SEPA transfer in Europe, you should be able to buy Bitcoin on either of them.

Coinbase is available in 30 plus countries and I think that you can use Bitstamp in most of the world if you can make a wire transfer over there.

Kraken.com and gdax.com may also work for you, as well as gemini.com and xbtce.com.

Finally, you have localbitcoins.com and wallofcoins.com.