Another vote for Trail Boss, if you're looking for bulk.
Personally, I haven't found that any of the other powders mentioned are cleaner than Unique, but I note that certain types of residue tend to bother some folks more than others. I'd rather clean off a light coating of carbon with a single swipe of a rag. But then again, if Yankee Candle had an "Alliant Unique Residue" scented candle, I'd probably buy it.
I'm intrigued by 4227 in the .38. I don't have any IMR-4227 / H-4227, but I've tested the work-alike XMP-5744 in the .357 and had promising results. (Just checked the Hodgdon online data, and yes, there are H-4227 loads for .38 Special with 158 grainers.)
If you're looking for very low residue, but can live with a little less bulk, you might want to try Hi-Skor 700X or Solo 1000. (The latter is difficult to find, and the loading data even more so.)