The short answer is: China. The long answer is, well, it's complicated.
After the bubble up to over 1,000 in 2013 which was largely caused by Chinese adoption (not Mt Gox fake volume as you may hear from others), the Chinese exchanges have played a central role in price movements.
- CNY: OKCoin, Houbi, BTCC
- USD: BitFinex, BitStamp, ItBit, Coinbase, OKCoinUSD
The USD exchanges have a useful index from Tradeblock called the XBX Index, a proprietary weighting of the major exchanges based not just on last price but on orderbook thickness and other components.
Watching the CNY markets is essential if you are going to be daytrading bitcoin, but the USD exchanges are gaining influence. 2015 has been a remarkable year compared to 2014 in terms of services available. And 2014 was amazing compared to 2013, which was amazing compared to 2012. You get the point. The future is bright for bitcoin services.