Best vibration isolation for speakers on carpet?

There is no “best” - that can only be judged by the ear of the beholder - you.

in general, there are two camps, coupled (i.e., using spikes) or decoupled (i.e., usually on an elastic support such as a spring or elastomer, but sometimes on a hard support such as DH Cones or a ball bearing solution.

Back in the day, after footers became a thing, conventional wisdom was to couple with spikes with the possible exception of speakers on a suspended floor over wood joists, where a growing minority started to believe decoupling sounded better.  Currently, people seem to be split as to which option they like best.  

Within each camp, the options range from very basic or DIY-type solutions to very expensive items that are often marketed as “the one best solution.”  

Spike options range from the simple hardened steel, threaded Ramset spikes used by Sound Anchor to large brass footers from the likes of edenSound or Mapleshade.  Townshend has the most popular spring solution, although simple springs can work too.  Herbie’s products may be the most popular elastomers, but there are a variety of silicone or sorbothane based elastomeric footer products available.  You will get many specific recommendations from the folks on this site.

@johnread57 - Those PMC MB2SE look sweet. The pictures on the manufacturer’s site show them with spikes. I used spikes under my heavy speakers for years (through carpet onto a concrete floor). One issue I had was that it seemed that all four spikes would never be exactly in contact with the floor evenly, there would always be some small rocking, regardless of whether they started out even.

For decoupling, you could try four Herbies Giant Fat Gliders under each speaker, which is what I use currently. For a less expensive decoupling solution, try four silicone half hemispheres. You cluld also try springs such as from Townshend or a less expensive variety.