Easiest:
-Levitation/Flight (I fly by levitating off the ground)
-Summoning People

It's funny, long before I knew what lucid dreaming was (I was just trying to remember to do cool stuff in the dreams, but didn't know it had a name and techniques for inducing it) I would always try to fly whenever I got lucid. I would try triple jumping (like in Super Mario 64) flapping my arms, etc. But would never success. Then some time after I found out about lucid dreaming and had some more experience I found that I could very easily levitate. Basically I would imagine a force pushing me up and moving me around. It became easier with every dream. I haven't tried flying and high speeds or altitudes, tough. I also use this "force" to move objects around.To summon people I just imagine or say out loud "X person is gonna be just around the corner) or something like that. It works most of the time save for some dreams where I can't do it for some reason.

Hardest:
-Summoning Objects
-Teleporting

Oddly enough, I usually fail at summoning objects. I use the same method as summoning people, for example opening a drawer and thing "X object will be here". The thing either does not show up or isn't exactly how I pictured it. I also have trouble getting to places. I have tried a few methods like spinning around but ended up in a different place. It was interesting tough, I spun around and began seeing shapes and using my imagination to "transform" then into things relating to the place I want to go. If I practice some I think I can start using this effectively.