How to find the best cards to power up your collection.
WHY?
You need power to:
1: level up and enhance your league
2: get more air drops
HOW?
To find the best cards to power up, we need to find the best CP(collection power)/usd ratio.
This means we get the most power for each dollar we spend.
1: Go to https://peakmonsters.com/
2: Go to buy section by clicking on 'Buy' tab or navigate to https://peakmonsters.com/market
3: Click on compare button
4: Choose Best CP/Cost ratio from the dropdown.
5: On this list you see the best CP/Cost ratio cards for this moment.
This information is based on Crypto Llama's video. 
Check his video for more info about collection power.