I find coingecko's ranking to be relatively objective and reliable at the present.


1     Binance
                  
2     Kraken
                  
3     Bitfinex
                  
4     Gate.io
                  
5     KuCoin
                  
6     Bittrex
               
7     Poloniex
                  
8     Coinbase Pro
         
9     Huobi Global
                  
10     Upbit
               
11     Bitstamp
               
12     bitFlyer
               
13     Bitbank
                  
14     HitBTC
               
15     OKEx
                  
16     Coinone
         
17    Coinsbit
               
18     BitMart
               
19     Gemini
            
20     BitMax
               
21     EXMO
               
22     Huobi Korea
            
23     Shortex
               
24     Bitsdaq
               
25     Zaif
               
26     Omgfin
         
27     Independent Reserve
               
28     FTX (Spot)

29     Bitkub
               
30     BTCMarkets