36 holes at Prestwick, on a lovely spring day, with informal lunch upstairs (the dining room was being refurbished), kummels for my partner and I left behind the bar by four gentlemen members with whom we got into conversation, a cracking game (which I lost), and then going out for a few more holes after tea, halving the 37th in three, and on returning to the clubhouse being asked by another elderly gentleman member if 'we were by any chance practising for the Amateur?'!
That has to rank very high indeed, with only perhaps a 50-hole day at Luffness New a couple of years earlier to challenge it, or maybe a blissful June day at an almost empty Brancaster (a visiting party had cancelled). But that Prestwick day was very, very special.
36 holes on one course will, for me personally, always trump rounds on two different courses in the same day (unless you are visiting, say, Saunton or Walton Heath for more than one day, and have the option of playing both courses twice within 48 hours). Not least, as others have said, so that you can have a proper lunch and one or two drinks, wherever you are, which for me is absolutely part of a golf treat.