My choices, with too much thought put into them...
BBC1 90s - this is tricky. The virtual globe is really nicely lit and has a lovely moody ambience that was a nice different direction to take the globe. On the other hand the balloon is more interesting and the fact you had multiple idents meant it retained its freshness. The best idents, however, were the initial launch idents in '97, because there the balloon was flown for real (as opposed to being CGed in later), and like a Storm Thorgerson album cover, it gains so much from being done for real.
BBC1 R&M - Skateboarders was a nice contrast to the rest of the set - sombre and graceful, avoiding the obvious route that many of the other idents took - that being 'Yo mate! We're down with the 18-25 year-old demographic!' Music Video was probably the worst offender in this category, and also unsuitable for about 90% of BBC One's programming - at least Tumbler fitted in perfectly with daytime television. (I do also like Capeoria and Tapdogs.)
BBC1 Circles - Kites and Mission Control are my favourites, the latter for nostalgic Doctor Who-related reasons. Windows was the blandest but even that had a cute soundtrack. Though all the idents were hindered by their being shortened down.
BBC1 Oneness - JESUS CHRIST GSGUG^Yw$!%£&@(!))
BBC2 1991 - Immediate favourite would be Firecracker, with close calls being Wave, Car, Neon or Diary. With the exception of Paper Cut-Out where nothing happens (but even that one isn't too bad), they're all consistently class.
BBC2 1997 - See above, except they dropped Firecracker and Diary to make way for the 2000 idents.
BBC2 2001 - Sticky Label was a nice then-unusual attempt to muck around with the logo and do comedy in an ident, while Formation has an iconic end shot in my view. Don't really have a worst ident, they were all consistently decent, if trying too hard to be funny on a channel that still had to show Newsnight.
BBC2 2007 - Mirror and Seascape were the best, Tagging was an interesting idea, Sunroof was the worst for being bland and not standing up to repeat viewings, washing over the viewer without really drawing your attention.
BBC2 2018 - all are decent, but Gritty and Punchy stick out as my favourites, while Illuminated was too simplistic for my tastes.