In June I did both Col du Galibier and L'Alpe d'Huez and loved both of them.
Couldn't be bothered with the Glandon, eh?
I actually think bringing non-UK climbs into this is a bit unfair, so i won't comment on my (v. brief) Alpine triumphs
I've loved a few climbs in Wales, but they're confined to the 2 long Audaxes I've done, so nothing original:
From the BCM 600:
Pen-y-Pass from Beddgelert
The A487 climb parallel to the south side of Cader Idris (heading NE-ish)
From the MC1K:
The road past Llyn Brianne to Tregaron (I'm sure the summit has a name too, sorry)
None of these are bonkers hard, but with gorgeous views. The Llyn Brianne road wins cause it's a lot quieter, although climbing past Snowdon is inherently quite exciting. I hope to try some of the other Welsh classics in the near future.
(I found Gospel Pass had a lower views:grunting ratio. The constant changes in gradient made it mentally a real slog. Enjoyed finishing it more than climbing it. It's a nice valley, but not as stunning as the ones above.)