Here is the spreadsheet we keep up-to-date with all the info we have about the game. It is sortable so that you can filter/sort monsters by any category you care to.
It also ranks all monsters by Health / Attack / Rest and Total Stat Average. There is also a column to compare each monsters individual stats to a cat of the same level. The cats were chosen for this purpose as they are generally recognised to be the most useful of the main island monsters.
There are also tools to allow you to calculate how much experience you need to raise a monster to a certain level, what level you can reach with a particular amount of boost materials, to compare any two cards head-to-head.
Link to public collaborative spreadsheet:
UPDATE: Due to recent changes by Google, you are no longer able to edit Sheets from mobile browsers. If you want to use this on mobile you will need to use the 'View in Desktop Mode' option or download the Google Sheets app and view it via that :/