runecraft isnt too bad to start, but the xp rate is terrible.
Summoning. you ahve to get all the charms
dungeoneering is terribly boring, even on the first dungeon
Question: In your opinion, what is the toughest skill to start training?
Answer: When it comes to pure, hardcore grinding - I'd say Slayer. Prayer would be the hardest skill to level, based on how much money it costs to do so. Training Prayer once you have the bones is quick and easy.
I've always really disliked farming. To me, a runescape skill should be a little more active and should yeild better rewards. If price is a factor of difficulty, then construction and summoning are hard to train too. But I love summoning
Ithink some skills should be harder, but if they are, they should be rewarding (like dungeoneering).
Summoning or Dungeoneering. Summoning because charms are untradable and therefore you can't just buy them and train Summoning easily- you have to kill thousands of monsters (probably while training Slayer) to get many charms. Dungeoneering due to the virtual need of every other skill making it so if you don't have the level to open a door your experience is severely reduced.