Fiends and summoners definitely look like best investments. Having fiends as max level in champion is incredibly powerful, while summoners.. well they're the key to doing anything in the game.
The only reason against investing too much in summoners that i see is the legendary airdrop summoners, but that's likely a few months away.
!1UP
I didn't even know there were going to be legendary airdrop summoners. If they are as overpowered and game-breaking as the untamed ones it's going to cause a lot of frustration to many players. It kind of feels like the regular Chaos Legion Summoners have quite a lot of power creep with 2 abilities.
It's going to be interesting to see the price evolution of the Fiend Legendary cards.