I quite like the Alderney case actually, though some details of the observation make me think they were actually seeing some kind of strange optical effect involving reflected sunlight. I've read Martin Shough's exhaustively detailed paper on the case (at
https://martinshough.com/aerialphenomena/index.htm) which does say a reflection of some kind is possible.
Edit to add: the supposed corroborating sighting at Alderney by a second pilot seemed a bit dubious in that he was looking back and seeing what he thought was a faint yellow object in haze, but it wasn't self-luminous as Bowyer's objects seemed to be; in any case it wasn't unambiguously the same thing viewed from a different angle.
The thing about my mental list of 'most impressive' UFO cases is it gets so heavily coloured by what are really just my favourite UFO cases.
But I'd say:
- Stan Hubbard's first sighting at Farnborough in August 1950, closely followed by his second sighting
- Michael Swiney and David Crofts, Little Rissington, 1952
I've no idea what this lot were seeing but something odd was going on.
In no particular order, some others I quite like are:
- Levelland 1957 (there are serious issues with the 'evidence' here, but I still think it might point towards some kind of as yet undescribed electrical phenomenon)
- The Egryn Lights, 1905, because who can resist a Welsh earthlights case from over a century ago
- Minot AFB 1968 (nicely re-researched in modern times at
https://minotb52ufo.com/, lots of multiple-witness chaos and possible misidentification, but the fact a bomber was asked to take a look at something on the ground puzzles me; also Blue Book had to resort to a 'plasma' to explain part of it)
- Cussac 1967 (probably a helicopter rather than a craft and entities, but oh well)
Ones I used to think were interesting and am now less sure are the Mansfield case from 1973 and the RB-57 case, which Tim Printy has explained quite convincingly; also perhaps Tehran 1976.