Which currency trading most and Forex session and transaction where most

According of IMF ( international monetary fund ) the USD traded mostly in the Forex market then GBP and JPY and EURO

USD traded about                   62%
GBP & JPY                            4%
EURO                                   2%
Rest currency                       28%
* (source IMF 2012)

So USD is the king in the  Forex trading market.

Trading Sessions:

The Asia Session (NZ, Australia, Japan ) open = 10 to 11AM GMT
The London Session ( GB and EU ) open = 7 to 8 AM GMT
The New York Session ( US, Canada ) open = 8 AM GMT
*note : during DST (day saving time ) it open 1 Hour ago.

The Forex market is acting as Over-the-Counter ( OTC) or interbank mean market entry electronically open 24 hours.

  Per day transaction of Forex market: 

New York stock  exchange about 22.4 billion $
Tokyo stock exchange about 18.9 billion $
London stock  exchange about 7.2 billion $