When I first started using an ultrasound cleaner I tried a dozen or so good quality off-the-shelf dishwashing liquids ($5-$7/24oz) mixed in a 5-7% solution, athen I tried a commercial automatic dishwashing liquid ($10-$25/45oz) and Tide laundry detergent ($10-$20/72 or 150oz), both in a 5-10% solution as well. I had very good results with all of them, but especially with the liquid Tide.

Then I tried Simple Green (about $10-18/gal) in a 10% solution and I was quite impressed with it as well, but when I tried Sharptek Solution 1831 ($ 40-$50/gal) in a 7% solution I was completely satisfied that this was in fact the best solution. Now I use Simple Green for day-to-day tool and small parts cleaning, and I use Solution 1831 HD or Tide when I really need a seriously effective solution to clean really heavy grim like diesel truck parts.

BTW: I also use Solution 1831 HD or Tide in my pressure washer to clean the shop floor, and both work really well.