Changing Times
If I had to answer this question a couple of years ago it would have been a definite US, by far, however, I have recently completed a couple of 'round robin' UK trips, and my perceptions may have changed.
Some of my trip was IFR, some VFR. Airfields in the trip include White Waltham, Dunkenswell, Perranporth, Sciliy Isles, Caernarvon, Haverford West.
Without exception I found a real welcome, good food, relatively cheap landing fees, parking, assistance for onward travel/stay, and everyone helpful - including transit ATC, airfield personnel, met facilities, et al. My view is that a lot of UK GA, is improving, modeling on US style GA airfields. The level of service is also rising, one could argue it might not have got any worse, however, the standard is high and getting higher.
The US is excellent, however, rising fuel may impact this, plus the NIMBY brigade chasing airfield closures, I last flew in the US last year, Los Angeles (Van Nuys), and noticed a slight downward movement in service, the flying however, was great.
I will be the first to bang on about regulation/maintenance/ATC issues, however, mu most recent trips have actually been very positive