We're using Castrol Syntilo 9954. It's the first full synthetic coolant I've used. We switched to it because of sump life. This stuff lasts and lasts. Just maintain concentration around 10-12% with our refractometer and a chart and it's good. We change it after a year just because our procedures tell us to (medical shop with procedures written for the old coolant). We had our supplier test it and it was still good.
I don't think I would recommend any other at this point. No smell, no rust (at all), and it lasts.
We also use a reverse osmosis system for the water. We got it at the same time as the coolant.