Elixir and planks share 1st place of being the easiest ones. Heavy ranged units have both great survivability and damage.... Elixir being a bit easier due to higher number of enemy melee units (I think, might need confirmation though).
Steel takes 2nd place. Light ranged units are your key units.. Nuff said.
Silk takes 3rd... even though you don't get the opportunity to strike first and/or kill almost everything before it even reaches you like in planks/steel/elixir fights are quite easy.
I've heard that gems and marble are really easy if you have dogs... Built my training grounds after gems tournament was over, will see how they do in marble tournament.
But that's if you have a lot of "fighters" in your FS. If you have a lot of "traders" who prefer to cater fights, then tips don't matter. Just stockpile necessary goods.
P.S. Our FS is going for 10th chest in Planks tournament.... Too bad that steel and planks are one after another.