Firstly, IPL was never meant to compete with International cricket. Thats why there is a restriction of only 4 foreign cricketers in the playing 11. The whole idea was to groom Indian talent, entertain and make money. And I guess IPL has achieved most of it. Where else a 19 yr old Sanju Samson will get to face Mitchell Johnson bowling at 150 KPH , or Akshar Patel get a chance to bowl to AB. These 19-20 year olds are just going to get better. Where else they would get a chance to bat alongside AB when he is whacking Steyn.
And not just Indian youngsters, even foreign born like Miller, Lendl would have gained so much.
Just the exposure to be around Indian and international pros and coaches is going to make so much difference to these young cricketers.
And its already showing, the way Manan and some of these guys have developed in last couple of seasons.
David Warner who has turned into a leading run scorer in Test cricket thanks to Viru. and this was Waner's own admission.
As far as the standards are concerned, there have been some terrific catching, fielding, batting and bowling. The standards of IPL have matched international cricket and better in some cases.
As far as grounds are concerned, there are smaller grounds in UK, New Zealand and SA. The ground size is not the reason for so many sixes, its the competition. Most of the sixes hit are over 75-80 meters which is good enough for most of the international grounds.
As far as English crickets are concerned, its due to ECB's restrictions. And its understood because if the stars go play for IPL means less eye watching English cricket means less advertisement means less money for ECB. Its unfortunate that both run at same time else guys like Stokes, Bopara, Anderson will get picked on day one of auction.
There are Indian and New Zealand coaches also

Going back to original question - I guess Akshar Patel is my pick. 62 overs, 17 wickets, economy of 6.2, very good for a 20 yr old. Just for comparison, SP Narine's stats - 60 overs, 20 wickets, economy of 6.01.
He can be a big threat to Jadeja if he can bat little better. He is tall and turn the ball, can be a great asset in test cricket. long way to go though.
T20 is batsman's game, so not going to pick a batsman