Exchanges with the best performing IEOs are Binance, Okex, Gate, Huobi and Bgogo to the best of my knowledge and you can choose from any of their upcoming campaigns. But that not withstanding, also do your own research on each project before making a buy
IEO is not like coins that we will advise people to buy the ones at the top in coinmarketcap, is not all top exchanges that are handling the issue of IEO very well to satisfaction, only Binance is the one that has satisfied people so far in this aspect, other ones are just doing try and error and getting the error in it.
People have complained about Bittrex, I have seen complains about Yobit IEO too and all are being verified, the worst is even smaller ones that are also coming up with IEO when the bigger ones has not succeeded in holding the ones in their capacity firm like the way Binance is doing.