The Fullmetal Alchemist franchise is widely considered to be a legendary series in the world of anime/manga and includes a uniquely impressive power system as well. To use these powers depends on one's knowledge and the Law of Equivalent Exchange: Such is what alchemy, or Alkahestry, is. While there are multiple amazing displays of alchemy throughout the series, there are some types of alchemy that are more powerful than others, and these either show their strengths in battle or somewhere else.
This list is a ranking of the top 10 most powerful types of alchemy used in Fullmetal Alchemist. If you haven't seen or read the series, be prepared for spoilers.
10 Envy's Transformation Alchemy
Envy is an interesting homunculus, simply due to his body. His (or her, as Envy is considered genderless) entire body takes the form of what looks a bit like a giant lizard (though much more disturbing) with human limbs and heads protruding from every direction. It's as if his body is a philosopher's stone given form - which is partly what he is, of course.
The most interesting thing about him is his ability to shape-shift into whatever he wants, which causes significant problems for the characters of the story (most notably for Maria Ross and Maes Hughes). Envy's specific transformation alchemy is on the list simply due to how much chaos he can bring by changing into just the right person.
9 Armstrong Alchemy
"Passed down through the Armstrong line for generations" (cited - literally whenever Armstrong opens his mouth), this alchemy is used by Major Alex Louis Armstrong. It's an interesting type of alchemy because of the fact that Armstrong primarily uses it to transform everything he touches into statues of himself - particularly of his face - though, who wouldn't, with his pure artistic ability?
The alchemy is powerful in the sense that it can pack quite a punch, and also, at the same time, confuse your enemies.
8 Chimeric Alchemy
This is one of the more unsettling types on this list. Chimeric alchemy, in the basic sense, is what Shou Tucker did to his daughter, Nina, and Alexander, their dog. It is the practice of combining two or more living things in order to create a new organism. This new organism is called a chimera. Chimeras, in the Fullmetal Alchemist universe, are treated often like things that ought to be pitied.
What makes chimeric alchemy so powerful is the fact that it is so difficult to actually do. Working with living organisms requires a lot of understanding of anatomy and cellular structure. The fact that it is even more difficult to return a chimera to its original form makes this alchemy all the more daunting.
7 Blood Rune Alchemy, Or Soul Binding Alchemy
This is the alchemy you seal with blood. It is the practice of binding a soul to an object or another organism. It's what Edward did when he called Al's soul back to the living realm. This alchemy is so powerful because of this fact. Ed's ability to bring Al back can be considered amazing alchemy, and the world of FMA even considers this so.
Because of the difficulty of this alchemy, there are only a couple of characters revealed to have had this done to them. Soul binding can also be used to make one's very own army, which is exactly what the Amestris military attempted to do behind the scenes to help prepare for the promised day.
6 Mustang's Fire Alchemy
Like this needs to be explained. Roy Mustang's control over fire is absolutely legendary. It can burn anything to a crisp in an instant. One of the most powerful displays of this is its repeated use against Lust. The fire kills her over and over until her philosopher's stone is used up.
Unfortunately for Mustang, its weakness is the rain; he can't transmute if his ignition gloves are wet. Though he didn't need them to kill Lust - a carved one in his hand was all he needed - this drawback puts it further down the list.
5 Fullmetal Alchemist: Brotherhood - Water And Ice Alchemy
While this impressive display of alchemy was an anime-only original for Brotherhood, it deserves a spot on this list, because water (unsurprisingly) can pack a lot of punch. Isaac McDougal, or the Ice Alchemist in the Brotherhood anime, starts off the series with a plot to destroy Central with massive icebergs.
McDougal's water easily defeats Mustang (though Mustang makes sure to get his revenge on the icebergs later in the episode), and the massive ice from the transmutation circles in the city make for a scary cold intro.
4 Truth Alchemy
What happens when you see the truth? You get the ability to transmute without a circle, that's what! The characters with this ability (like Edward and Izumi Curtis) seem to have extra powerful transmutations.
All they need to do to transmute is slap their hands together. This extra time saver makes each of them that much more of a threat. If only Edward had learned the truth for making himself taller...
3 Scar's Alkahestry & Alchemy, Combined
Created by his brother, Scar's alchemy is something truly amazing. His right arm destroys, while his left arm creates. Since his transmutation circles are tattooed neatly on his arms, it's easy for him to use them to quickly take control of a fight.
When Father turns off everyone else's ability to transmute, Scar can still use his techniques since the tattoos draw on both the ideas of alchemy and Alkahestry. It means he can still keep his alchemy fighting prowess even when homunculi are on the loose.
Or specifically, Mei's. Mei is revealed to have part of the key to saving Amestris as a whole - her knowledge of her home country's version of alchemy - Alkahestry. Alkahestry's suitability to medical practice and ranged transmutations is exemplified in Mei.
This type of alchemy is so powerful due to its variety of uses, and it doesn't hurt that it doesn't use the same energy source alchemists use in Amestris, either. Like Scar, she can use her Alkahestry as she pleases in Father's presence.
1 The Philosopher's Stone
It's not always a stone, but this item is powerful. The real-life philosopher's stone was rumored to make an alchemist rich beyond compare. The fictional philosopher's stone can help an alchemist seemingly do transmutations without equivalent exchange.
While this isn't true, of course (as the philosopher's stone uses human souls), that doesn't discount its power. A philosopher's stone can grant a certain extent of immortality, easy transmutation, and regeneration. It would have been perfect if it weren't such a terrible thing.