Just one type? I have uses for multiple types.
In general I prefer SFX over ATX, since we've gotten to the point an SFX can power pretty much any mid to upper mid range systems, the only remaining use for ATX really is only the crazy rigs with multiple GPUs and CPUs.
When it comes to smaller rigs, though, I prefer the PicoPSU, since it minimizes both the space taken up, and reduces cabling by quite a bit which can be just as critical. I would prefer to pair it with an internal AC-DC converter, though, but can't find many good models outside HDPlex's offerings. I do wish they could cram just a BIT more wattage on them, though so higher power rigs don't have to turn to the HDPlex and all the extra wires that means.
Flex ATX is a solid choice for a decent middle ground: smaller than SFX, but still a single, easy to use, standards compliant box, and with 4 and 500 Watt units starting to appear on the market, may be a suitable replacement for SFX.