mSATA would probably be the best but those drives are difficult to find these days. Short (2230/2242) SATA would be next best. These are mostly OEM drives but there actually are some retail ones that may be a bit sketchy. Example 1 and Example 2.

Actually, I take that back, as UFD drives are pretty efficient as they forego the bridge chip. This would be U17/U18 or SM2320(XT) based drives and there are even some in a smaller form factor (basically USB drive). You stated M.2, though, so I guess these do not apply, since at that point you can just go with a USB flash drive with the performance limitation.