2015:
The most unexpected transfer of the 2015 summer window was that of Petr Cech moving from Chelsea to Arsenal for £10million. It was highly surprising at the time that Jose Mourinho would let any of his (then) Chelsea players move to a club which is seen as a direct rival.
2016:
The biggest story from the 2016 summer transfer window was probably the staggering amount of money spent by clubs in the EPL. The total spent by top flight English clubs was a remarkable £1,175,230,000 during the window.
Manchester United of course broke the transfer fee world record when they re-signed Paul Pogba from Juventus for £89.3million. Other hefty transfer fees were those paid for John Stones (Everton to Manchester City for around £50million including add ons) and Granit Xhaka (Borusia Mönchengladbach to Arsenal for around £35million).