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)