The difficulty changes for humans\elves at different stages in the tech tree. For example, in my chapter and with my wonders, my training ground is quite a lot faster than my barracks. So, the last planks tourney is one where its helpful to use a lot of orc strategists. I also finished the 3 star tech upgrade for strategists recently (often a star upgrade makes that troop type more efficient to use for a period of time). In short, between the tech unit upgrade and the faster training grounds, planks was a fair bit easier for me this time than it was 9 weeks ago. Other examples of how things change include humans having a big advantage in gems and marble because their mages are soooo good (best troop type in game), and elves in any tourney that uses a lot of heavy ranged (golems being better than human mortars). On balance, humans have a better set of troops than elves though, the balance between the 2 isn't quite right.
So, the the following list has some qualifications, and as wonders and techs change so will the difficulties of various tourneys. But, as a rule of thumb, from easiest to hardest the tourneys imo:
Steel
Elixir (at least for elves with their golems)
Gems (would be #2 for humans imo)
Planks
Crystal
Silk
Marble
Dust (although this gets a LOT easier with a decent level victory springs)
Scrolls