Creatine, being the most popular ergogenic (sports-enhancing) supplement in history, comes in many forms. Some of these forms are scientifically found, whereas some are the children of an incestuous relationship of marketing and theory.
A full list and critique of different forms of creatine can be found in this subheader, Different forms of Creatine and Creatine Supplements.
Assuming Creatine Monohydrate (most frequently used in studies) is the standard by which to compare, no form of creatine has shown to be more powerful or potent.
Creatine Magnesium chelate may be a bit promising for its ability to prevent water weight gain at low doses. Creatine pyruvate also seems to produce higher blood levels of creatine, but this hasn't been shown to increase performance in any way.
Creatine Citrate and Creapure (Micronized Creatine), although just as potent as Creatine Monohydrate, are more water-soluble and may be of interest to avoid the clumping in water. This is similar to Creatine Nitrate, which currently does not have any legitimate scientific evidence to support the claims of superiority over Creatine Monohydrate.
Buffered Creatine (Kre-Alkalyn) and Creatine Hydrochloride (Con-Cret) both are negated by stomach acid, and turn into the basic creatine molecule. They are not worse, but not better either (assuming the dose is the same). Creatine Hydrochloride may be more water-soluble than Monohydrate as well.
It should be noted that the form known as 'Creatine Ethyl Ester' is actually much worse than creatine monohydrate, and degrades almost completely into the metabolite creatinine in the intestines.
There are no significant differences between powders, tablets, or capsules. Capsules and tablets are just vessels for the powder.
Overall, Creatine Monohydrate is the best 'bang-for-you-buck' form of creatine as the others tend to carry higher prices on the labels. That being said, other forms may have benefits that are not related to the creatine molecule itself but due to solubility.
Those with stomach cramping with creatine (which may be due to creatine monohydrate forming an insoluble precipitate in the stomach) should consider a more water-soluble form of creatine.