Even though I do agree with LeGaulois's comment ^{[bestchange part]}, the following links ^{[that I've gathered over the years]} might come in handy ^{[not exactly a monitoring platform, but close enough]}:
^{Take note: I'm not vouching and you should never fully trust any of the listed exchanges [a lot could change in a short period] from such platforms [always DYOR].}

• CoinGecko
• CrypTwerk
• BlockSpot