It's been said, but it's kinda hard to pick. So many awesome ideas! So I guess the absolute factor would probably be either whether this power is the one I WANT to get or the one that best suits who I am as a person, my personality basically. Or my favorite balanced power.
If it was the first, there is no question about it: I'd want the power to have the inconsequential ability to alter any aspect of reality I wished, including my own abilities. Not only would I get more powers with this, allowing me to do basiclaly everything (and the first I would get is immunity from any stealing/copying/nulling powers), because of the inconsequential aspect none of it would have unforseen consequences, as all consequences would be only determined in accordance to the exact aspect I wished to change. If I made a ton of money, the ability wouldn't, say, end up inflating prices.
But that's a little boring since well, it's obviously a power designed to be the best power ever. The second option I think, is much more challenging and interesting. Faulty copying, merging, shapeshifting, regeneration, toxins, monstrifications, enhancing, all come to my head, though I think that ultimately, the kind of ability that would suit me the most would be summoning. I'm cowardly and often lazy, but I'm also very creative and have a strong ego. So I think some form of summoning would match me perfectly.
As for my favorite balanced power, that'd be shapeshifting. @Daisie already made a great job of illustrating why , so go read that XD
Also, if anyone wants to, I have a thread in which I come up with superpowers for ya peeps. Check out my signature in "Idea makes you a superpower!"