In center-fire calibers, the lightest force needed to rack a slide will probably be found in locked-breech 380 pistols (i.e. Sig P238 and S&W Shield EZ). It takes little effort to rack the slide of a pistol that operates on the recoil of such a mild cartridge.
Considering 9mm, slides on full-size guns will generally be easier to rack than their compact or subcompact counterparts. As previously noted, the Beretta 92's slide is generally considered easy to rack, probably at least partially because its recoil spring does not have to be as strong to handle the reduced mass of the open-top slide design. Also as previously noted for hammer-fired designs, the force to cock the hammer can often be separated from the force to rack the slide (e.g. overcoming the recoil spring). Those forces cannot be unbundled in striker-fired designs, which often have heftier recoil springs.
Considering 9mm, slides on full-size guns will generally be easier to rack than their compact or subcompact counterparts. As previously noted, the Beretta 92's slide is generally considered easy to rack, probably at least partially because its recoil spring does not have to be as strong to handle the reduced mass of the open-top slide design. Also as previously noted for hammer-fired designs, the force to cock the hammer can often be separated from the force to rack the slide (e.g. overcoming the recoil spring). Those forces cannot be unbundled in striker-fired designs, which often have heftier recoil springs.