If you're talking cap as in an amp's tone caps, then I defy anyone to pick something
better than NOS mustards for Marshall circuits.
There are equivalents, such as Sozo Vintage Series caps, however, my experience and preference is NOS mustards.
Actually, George is building me my next Metro right now, and he will use NOS mustards. The use of these will get the build as close to old school 60's Marshalls as is possible. With some NOS 12AX7 Mullards in the preamp and some Mullard EL34's in the power amp, is there any question that the amp will sound anything less than brillant?
For guitars try the RS Jensen paper-in-oil caps - they are excellent in helping achieve that FAT Les Paul tone! Mmm mmm!
Cole