What is the best exchange to invest in WAVES? I only have accounts on Poloniex and Kraken and no WAVES there 
Buy from bittrex https://bittrex.com/Market/Index?MarketName=BTC-WAVESLiqui.io is also nice exchange and Bittrex is one of the best exchange. So you can buy Waves from both exchanges and can make withdraw instantly. I have good experience with them and currently using them.
Or use the waves DEX. More volume than liqui but not as good as bittrex(yet)
DEX volume keeps increasing. It's reached $500k last 24 hours.
https://coinmarketcap.com/exchanges/waves-dex/
and with the new assets ZrCoin, EncryptoTel, MobileGO and more, for sure great potential volume.