I love them both dearly, but after a recent re-watch its confirmed, B5 is just outclassed by DS9 in nearly every way. Budget, set design, acting talent, writing, etc..
Yeah re-watching DS9 during the pandemic then
attempting to re-watch Babylon 5 showed the disparity in quality was actually far, far greater than I thought it was in the 1990s.
As you say, DS9 outclasses B5 on every level, and almost from the get-go too. S1 and S2 of DS9 are also better than people give them credit for, even if there is the odd questionable episode (c.f. Move Along Home - but I'd kind of challenge you to find any 22-26 episode per season hourlong show - really 43-48 minutes but that's beside the point - that didn't have the odd bum episode).
DS9 could easily become overrated, but right now it remains, I'd suggest, very slightly underrated by geekdom as a whole.
TNG is slightly overrated, but people have acknowledged the weakness of S1 and S2 for a long time, like, close on 30 years, so not hugely, and what's shocking is that we haven't seen wildly better SF shows since.
I think this is key in why the DS9 and TNG are still so well-regarded - they haven't been "beaten" by another similar show (or even videogame or whatever).
Returning to the original format:
Underrated:
Travelers
Yes its a Canadian SF show. Yes it stars the Will and Grace guy. But it is honestly a hell of a lot better than a huge number of better-known and better-regarded SF shows.
(Runner up: Supernatural - only talking about as "geek media" - it's popular generally, but sneered at by geeks, and frankly, whilst it is trashy as hell, and the first two of the fifteen (!!!) seasons are not amazing, and indeed there is a whole lot of "filler" and dodge eps, I feel like geeks are sneering at this more because of misogyny and contempt for the mainstream and so on than because it is worse than stuff they watch.)
Overrated:
Many major 1980s and early 1990s fantasy novel series
Specifically The Belgariad, Thomas Covenant, Shannara, Pern, Dragonlance, Paksenarrion, just not well-written, not clever, not interesting, don't have much, if anything, to say, and don't remotely hold up when compared to both earlier and later fantasy novel series. Frequently staggeringly sexist or even misogynistic and rapey in ways people really play down. In Britain they were jokingly referred to as "extruded fantasy product" (like extruded plastic or meat), because they were so bland and awful, but for whatever reason, they got kind of eaten up, and people still try to convince people today to read them. Wheel of Time kind of
just barely escapes being put with these - its very close to that bad, but just kinda manages to edge into mid. There are a lot of others which are kind of mid but not bad, like the Riftwar series or Legend/Drenai, or which are kind of not really overrated like the Black Company, and some stuff which is exactly rated correctly as amazing like Discworld, but bloody hell this was a dark era for fantasy despite some incredible numbers of novels being churned out and even printed and sold. I think stuff like the Belgariad and Shannara and Pern being so successful are honestly the main reason fantasy had a very bad (much worse than now) reputation among literary critics in the 1990s and 2000s, and took a while to shake it to any degree. Seeing people buying these really awful turgid vapid books cannot have helped. Also I'm mad because I think these being so bad yet continuing to be recommended leads to people reading them instead of extremely important and influential authors like Moorcock, or staggeringly good authors like Wolfe. There are also at least a couple of other bad 1980s and early 1990s fantasy series which aren't as bland and dumb as those initially listed but still really bad for other reasons, note.
(Runner up: Malazan. Jesus wept what a great concept that over maybe 7 books steadily and determinedly dissolved from cool into trash-tier near-parodic fanfiction of itself.)