AS LONG as you take care about flammable fumes, I recommend cheap paint thinner.
This works very well to degrease and clean, drys quickly, and and is "thin" enough to get into areas some thicker soap-based cleaners don't get to as well.
For small parts, I put about one inch of plain water in the bottom of the tank, and put more volatile and flammable solvents like lacquer thinner in small glass, metal, or plastic bottles or jars.
Put the jars of solvent in the tank, and the ultrasonic waves are transmitted through the water and into the jars.
This allows the use of more flammable solvents by reducing the amount used.
Lacquer thinner works VERY well to degrease and clean parts, and it also has a slight "brightening" effect when used on brass.
The only reason I don't use it instead of the paint thinner is because of it's much more flammable nature.
If you want to go with a water-based cleaner, most any liquid soap cleaner mixed with hot water is good, BUT..... Be SURE the cleaner is not harmful to some parts.
As example, Simply Green will seriously damage aluminum.
If you use a water-base cleaner, after cleaning flush the parts VERY THROUGHLY under a hot water faucet. The hot water will flush away the cleaner, and heat the parts so they will dry faster.
In any case, after cleaning and rinsing, use compressed air (if you have it) to blow off the excess, then use a hair dryer to warm the parts until dry.
NO HEAT GUNS, they can get parts TOO hot.
After cleaning and drying you MUST be sure to get all parts and all hidden areas coated with a rust proofing lube, since ultrasonics remove ALL lube, including from tiny hidden holes and cracks where ordinary cleaning never gets to.
Even stainless steel needs a thin coat of a good rust-proofing lube like CLP Breakfree.
Some other pointers:
Ultrasonics work better with warm solution.
Either use hot water, or let the cleaner run until the solvent is warm before removing parts.
Ultrasonics do little for leading, and nothing for copper fouling unless the solvent itself works on it.
Usually, you still have to clean the bore and chambers with a rod and brush.
Ultrasonics will remove painted sight and safety markings, and MAY damage Tritium sight inserts.
KEEP YOUR FINGERS OUT OF THE TANK.
Ultrasonics and bone marrow don't mix well. This is something that happens over time, not instantly.
Lids or covers hold down the evaporation of solvents and reduces the risk of fire, as well as solvent smell in the house.
You help cleaning along by pulling parts out of the tank and brushing with a solvent-proof toothbrush.
Hang parts in the tank with wire, or use a basket.
The cleaner works better if the parts are not laying on the bottom of the tank.