What contact cleaner do you recommend?

Save your money and by that new piece of music you have wanted.

Not to say that you will not convince yourself that you hear a difference. Just plug in and out several times every six months if you must. If your connectors fit snug and are of good quality you will have no issues. Is your system in a room with high humidity,(over 55%)?

I had excellent results with Xtreme AV Quicksilver Gold silver liquid with gold content. No problems with degradation, etc. Superb for all connections including tube pins, power cord plugs, fuse end caps, etc.

They all degrade over time.

Kontak works the best for me,

As far as a cleaner, I use 99% Isopropyl alcohol. I no longer use any metal bearing pastes or liquids, as the carriers in most cases tend to dry out into a gummy mess (especially with heat). I find Caig DeOxit to be moderately useful when cleaning switches and pots (things that move or will be moved) but fairly useless on stable objects like RCA connectors as it tends to adversely affect the sound.

As a contact enhancer, I use Stabilant-22 concentrate mixed with isopropanol as the carrier, which I think is considerably better than Kontak. YMMV and all that.