Like it was stated Bittrex is hardest to get on but most reputable IMHO. Cryptopia is probably the easiest and most respected for customer service with upcoming alt-coins. Polo is picky about what coins they list and Coinbase will NEVER add this coin. Id go with paying the DOT and get it listed on Crytopia so investers can see interest in coin. No intrest no Bittrex!!
Cryptopia is fairly cheap to list a coin $25,000 – $50,000. Have a think about Cobinhood. Cobinhood has a trading engine capable of 1,000,000 transactions a second. All the other exchanges have fallen over under volume – also Cobinhood will have no trading fees (generates income form leverage trading x10 and ICO floatation)