Capacitor rolling.

In my view, it’s technology first, materials second, brand third.

Technology for the best caps, IMO, is either metallized dielectric (e.g. aluminum deposited on teflon), or film and foil (a layer of metallic film and a layer of dielectric). Film and foil is far superior. Then there is air gap, which consists of plates of metal separated by air, but that is limited to small values.

Materials mainly used are aluminum, tin, or copper for the metal, and paper in oil, polypropylene, styrene, or teflon for dielectric. Tin or copper can be soldered, and seem to me to give a smoother sound. Dielectrics seem to affect the sound as a function of dielectric constant: clarity improves as dielectric constant goes down. That puts styrene and teflon at the top.

That said, I find that styrene sounds a little more neutral, and teflon a little edgier. Air gap and vacuum are the very best, but very bulky and very expensive.

Last is brand and details of their process. Thickness of metallization is critical to the sound of metallized caps. More is better, but good luck in finding that information. MIT has a patented method of connection, which makes their multicap line sound better to me. Dueland is very close to an optimal configuration, but you have to pay for it.

I use a lot of MIT Multicap RTX styrene and tin f&f capacitors when I can’t use air gap, as they suit my system and my tastes. YMMV

Good discussion here and as expected different choices of capacitors preferred depending on the particular poster’s taste. Some like to use the term "neutral " and others such as myself use/prefer the term "natural", semantics. Natural is what I seek, in other words,  organic and realism. I’ve had the opportunity to use some exceptionally high quality capacitors. Just my opinion based on listening and acknowledging my opinion is no more or less credible than anyone else.

1.Duelund CAST copper foil.
1a.Jupiter copper foil.
2. V cap tin foil and Teflon.
3. Sonicap Platinum.

This is simply my preference based on perceived natural character. In my opinion all of these capacitors have an innate sonic signature which is unavoidable. Absolute neutrality doesn’t exist. We all choose what we believe best suits our own objective and listening criteria, as it should be.

I won’t argue over what’s the "absolute " best as this is pointless and unprovable. I accept that this is purely subjective by its very nature. One man’s neutral is another one’s thin . One man’s natural is another one’s rich. The debate goes on into eternity. Trust your own ears and you’ll be fine. Accept the fact that others will disagree with you, that’s okay.
Charles

Tubes, I have been told that some offerings, which I don’t like, require hundreds of hours of break-in before being listenable. I won’t subject myself to that. Then there’s the sneaking suspicion that one is simply adapting and learning to accept an inferior sound.

I have tried both RelCap and Solen teflon f&f caps, and find them to sound similar, for values ranging from 0.022 to 0.82 uF. I tested them in a break-out box beside comparably valued RTX, and found the teflon caps to be a bit bright, RTX neither bright nor warm. The best teflon caps I have heard where made of monolithic copper and skived teflon tape, a DIY project.

None approached air-gap capacitors - by which I mean, metal plates separated by air, no other dielectric. They are hard to find, large, and rarely above 2000pF, but they are the gold standard. Air-gap and vacuum. Surplus Sales of Nebraska sells them, both variable and fixed. For that matter, you can make your own.

But once you hear them! In fact, it may take you an hour to take in and understand all the distortion that you’re not hearing. Which is why I use them when I can.

Another vote for Duelund’s Cast Copper PIO, a couple values of which Parts Connexion currently has on sale: (http://www.partsconnexion.com/duelc-specials.html?utm_source=getresponse&utm_medium=email&ut...) The Cast Silver maybe more transparent, but I couldn’t afford that show’s entry fee.  Swapping the 10uF Solen caps in my B+ circuits, for Clarity's 630VDC  MR Series,  cleaned things up even further.