The Easiest:
1. Flying (probably closer to a semi-controlled float than actual flying)
2. Finding/summoning people (though after waking, sometimes the person I found wasn't really the right person, but in the dream it was?)

The Hardest
1. Navigating to a specific place (always get lost or end up in the wrong place)
2. Orgasms (having sex is easy enough, but things tend to get too unstable/wonky for the climax. makes me sad).