Easiest:
1) Flight
2) Conjuring elements or objects

Hardest:
1) Finding or summoning specific people
2) Traveling to a specific place

I don't know why my dreaming mind is so obstinate, but I have spent whole dreams just looking for particular people or characters -- whether fictional, historical, or even DCs I've met before -- and all I get is evasions, obstacles, and excuses. So much for having a dream guide; I don't think I've met the same DC twice ever. Maybe it's because I'm just not that much of a people person in waking life, I don't know. I can summon any kind of animal easily enough, and conjuring objects or elements usually works on the first or second try, but people I hope to find remain absurdly elusive.