I want to know in general what locos have the better sound systems. By better I mean the most realistic.
In that case the most important thing is to match the sounds recorded on the decoder to the unit it is installed in. I have a PCM F3 ABA set that the sound quality is ok but it is wrong. There is obviously a turbo charger (567D) rather than a roots-blower sound (567B) that this particular set of units had, the horn sound was wrong (model had a Nathan 3, and the sound unit is Wabco).
In models they often also screw up sounds of units with different cylinders. A 567 is not a 567 is not a 567. For example I am fairly certain the sound that BLI originally used in their E7 units was a 16 cylinder 567C instead of two 12 cylinder 567As as it should be. ... sigh.
The way to get realistic sound is to research the model that you have and then go hunting for the sound to match. That is actually much easier today than it used to be because many of the modern sound systems are programable. That is when the proper sound clip is found it can be uploaded into the decoder.
An EMD SD40 from the factory had a 16 cylinder 645E3 prime mover that was turbo charged so it will whine a bit. SD45-2s had the same engine. These should be very common sounds. Look for decoders loaded with "EMD 2nd generation" and odds are it will be right for this loco.
As far as other realisms go - louder is not better. Loud blaring sound hides its true quality. Put the loco on the track get down close and listen as if you were standing that close to a real loco. Adjust the sound accordingly.
Finally the sounds one can find on the internet will not sound the same when they are put inside an on-board unit. Remember a computer can have some really nice big speakers an HO locomotive cannot.
I don't believe you will be disappointed with Soundtrax Tsunami, QSI, or Loksound decoders. Do note that the Loksound use a high impedance speaker (not a normal 8 ohm like other systems) so they could be harder to find and there will be less selection. I have no Digitrax sound units but have read mixed reviews on them.