The ability to instantly teleport came to mind as I was about to get in the car and drive 6 hours... I just really really lazy sometimes.
Superman's super strength harnessed from the sun would actually be a much better super power.
My concern with this sort of superpower is that it might suddenly stop working and I would forever be stuck in the form of, say, a goat. It's a problem I have with superhero films; Spiderman, for example. One moment, he's looking with disbelief at these webs suddenly shooting from his wrists; the next, he's swinging wildly and carelessly across the Manhattan skyline. Surely a learning curve is required and, more importantly, a period of building trust in one's new-found arachnid-abilities.
Anyway, I'd fancy the ability to nudge time back and forth as with a wind-up watch, but I suspect we haven't the constitution to truly understand on what slender threads life and fortune hang without losing our minds.
Yeah, Animorphs from back in the 90s had fun with that on their character "Tobias". Animorphs was pretty out there with their shapeshifting, aliens, and ear slugs.
As for the time nudging thing, both Twilight Zone and Clockstoppers had fun with that premise (and naturally The Simpsons parodied it). It's a fun concept, but they always poke at the "what if the watch breaks" or "what if you age while time is stopped" angles.
I was more inclined towards teleportation before (teleporting martial arts is an awesome idea), but shapeshifting where I adopt properties of the form being taken's been sounding more appealing lately.
Being able to look like anyone or anything has way too many creative applications for affecting the world around you without any worries of accountability, plus shapeshifting would allow the immediate ability to heal one's own wounds (self-healing). It takes the benefits of a few power choices and slaps them all together if you know what you're doing with it.
Mystique, Clayface, Loki, Mr Fantastic (technically), Ditto, Shang Tsung, T-1000...
It's a great powerset that gives an abnormal amount of room for creativity. It's one thing to disguise yourself, it's another to be able to outright reshape yourself into anything and adopt it's properties.