The real limitation on the shapeshifting is the cleverness of the shifter. Plastic Man doesn’t function as a top-tier super because it generally doesn’t occur to him to use his powers in powerful ways. Someone more contemplative (and more generally knowledgeable, perhaps) could come up with numerous power stunts to effectively mimic or replace other powersets. I flatter myself to think that it would be substantially overpowered in my hands. I just wanted to see if anyone would call me on it. 
Another potential limitation is whether you can gain or lose mass or not, and whether/how far you can fudge densities to compensate for this. “Wonder Twin powers - activate! Shape of…black hole!” has a huge difference in effectiveness if you can’t modify your mass.
“Anything”, though I would agree that black holes stretch the point. So to speak.
(Even a low-mass black hole could be quite dangerous under the right circumstances, I think.)
I want the power, knowledge and insight to change any living entity I can touch at any level (from the atomic level to the macro level). This would let me alter DNA or modify physiology in any way I want, and I’d have the knowledge to know what needed to be done to achieve whatever it was I wanted to achieve and the insight to be able to look into a living entity to see what might be wrong with them.
I figure with this power I’d be immortal (I can, after all, touch myself and I’m living), and make others immortal, could cure all diseases, heal any not immediately lethal wound or issue, and alter other living entities in fundamental ways. Perhaps create plants that can absorb and store more carbon or other pollutants. The possibilities are endless.
Oh, I don’t want you laid out; I just want you on my team.
I’ll take the ability to jump to first place in any line. Then use it on this thread and take the power I want that someone already has!
The ability to maintain my body temperature at precisely the level I want. No more cold feet, no more uncomfortable sweating on hot days.
Swimming with penguins, winning bar bets, the possibilities [del]are endless[/del] are some!
The Homeotherm!
I would like to be able to eat anything I want with no health consequences whatsoever.
Good-bye, heart disease, high blood pressure, gout, and diabetes!
Turning into any animal is what I came here to say, so since nobody has explicitly said it as far as I can see, I want it.
Turn into a bird and fly almost anywhere. Turn into something small and eavesdrop on people. It’s close to unlimited powers. I’m sure I can find some day to profit of it, too.
It would be, but not for the reason you think. A black hole with the same mass as you is exactly as dangerous as you from a “sucking things in” standpoint since it produces exactly as much gravity as you do, but it would also evaporate basically instantly in a burst of extremely hot hawking radiation.
So turning into a black hole would be a very efficient way to go out with a bang. How big and hot a bang? Err, I’ll leave that to actual physicists.
I will be Doctor Do-Over.
Anytime something goes really wrong, all I have to do is choose to go back to some point in my relatively recent past…like a computer restore point…and use my knowledge of the coming crisis to fix it.
I mean, you could literally do the same with a small drone carrying a microphone right now. Just saying.
I can’t ride a drone if I want to, say, fly a state over!
Damn! I came into this thread to claim a Five-Minute Do-Over! (I think I might even have posted it someplace here quite a while ago.) Oh, well.
So I’ll go back to an earlier thought: Submicroscopic Vision, all the way down to reading DNA/proteins/whatever. Fairly boring, but useful for ID and probably lots more.
The power to compel truth. If I’m physically present; a witness, politician, preacher, businessman, etc… will be truthful regardless of notes, teleprompter, powerpoint slides, or aide attempting to snatch the microphone away.
I’ll call my superhero, Mueller-man.
I was going to take the power to heal anyone but it looks like Grim Render has that covered so I’ll go with the power to replicate anything. If I have some gold nuggets, say, I could multiply them umpteen times and melt them into ingots. Or I could go to a poor, drought stricken area with a truckload of various supplies and create all the food and water the people there needed. I could even create smaller, simpler houses, I think, but McMansions would be beyond my power.
I’d stay away from replicating anything with specific identifying marks, such as paper money because someone will probably notice if there’s a whole lot of hundred dollar bills floating around with identical serial numbers on them. Ditto with one-of-a-kind objects such as the Mona Lisa. There’s no need to cause all sorts of suspicion and confusion by opening up that particular can of beans.
Absolutely anything at all that is true, they will get convinced and stay convinced. It doesn’t matter what it is. They won’t have the option of not believing. But if what I say is false or only partly true, the false parts will be null and void.
But replicating actual beans would be OK. More like not opening a can of uniquely identifiable entities. 
The ability to create simulations in which I can put life forms. They will disappear from the real world physically. All my simulations must have a way out, a goal the life form has to accomplish, a story line to complete. I can always release them earlier or assist them, but never trap them forever e.g. make the goal impossible to accomplish.
After exiting the simulation the being will be physically unaged.