While I'm pleased with all of my components, I'd say it's a toss-up between my Outlaw 755 Amp and SVS 20-39 PCi Sub.
The Outlaw has made a substantial improvement over the on-board amps of a couple of quality mid-fi receivers I have been using for a pre/pro, particularly when listening at higher volume levels and closer to reference level.
The SVS 20-39 PCi has provided more than enough tight, solid bass response for a fairly decent sized room (27x14x8). While I've considered a model upgrade, I really think if it weren't for upgradeitis, I could use this sub in this listening environment from here on out...
If I took either of those out of the mix without a solid substitution, there'd be a big hole in performance... -TD