The D&D angels (even before they were called such in AD&D) have an iconic and multifaceted role that hasn't been eroded at all.
Solars have at times been hinted at being powerful enough to be Powers in their own right, if they weren't humble enough to serve gods or serve good instead of their own ambitions.
Even planetars despite being probably better than pit fiends and balors, have zero evil analog.
Monadic, movanic and astral devas don't even live in the celestial planes but keep watch throughout different areas of the multiverse.
Angels aren't even the good version of devils/demons, not just because the latter don't serve evil deities (usually) and exist in far greater number... but because there are actual frickin mirror-race celestials, or sometimes celestials anyway, archons (which finally made it into 5e, hooray), guardinals and eladrin.
So the D&D angel is a unique, and extremely mysterious, being.
Sometimes an angel falls, but evil gods aren't served by fallen angels.
Sometimes an angel falls and becomes an archfiend, but sometimes evil angels remain celestials.
There is nothing that isn't cool about angels in D&D. They are rare. They are few. They are monsters, but not monsters that usually threaten you. Because of this, they are not monsters that need to be beaten, or beatable. Solars used to simply kill you with an arrow with no save if they wanted to, which I didn't view as great.
They invoke a sense of wonder and awe that isn't dependent on whether or not they serve the very fallible gods and oft-capricious gods of good. And yet, they are simultaneously wise and hubristic. In 5e, once dispatched on a mission by a god, an angel can't be reigned in and doesn't question its own innate sense of right and wrong. Not all good angels are above politics, either.
There is seriously a lot going on with D&D angels, and a hell of a lot going on by good monster standards.