What contact cleaner do you recommend?

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.

I found that both Walker and Xtreme degrade significantly over a short time. They do make a very positive initial effect. Personally, I would avoid either.

I do recommend Walker Talisman and Xtreme CD cleaner. Those are great products.