which preamp sounds most transparent?

A passive line stage.

Only problem is, you can't control the volume. If you use the digital volume control that is the typical fare, you can expect it to sound worse than using a preamp.

The other problem is that not all CD players can control the interconnect cable, which is an essential part of the job that a preamp should do.

John is right. Others will have differing points of view. It seems that you are looking for a neutral pre-amp. I have been taken to task over this before since some people regard neutral as synonymous with flawed, but I think Klyne pre-amps are very neutral. Or in other words more transparent.

Try an autoformer based pre. This may be the most transparent, but may not give you the drive it turns out you like.

H-Cat 12B with Wavefront Timing Control. Incredible preamplifier.