A while back when I was looking at the gen 4 legendaries to make a list of pokemon to prioritize getting candy and luckies for, I sorted the unreleased sinnoh ones by weaknesses.
Lake trio- weak to everything we use to raid mewtw/deoxys/cresselia
Regigigas and dialga- Countered so well by fighting, (for dialga, ground types will need weather boost to stand out)
and Palkia- Much like with dialga, one counter (dragon) is near always supreme, with the other type (fairy) needs 2 conditions for it to be better (cloudy + draco meteor)
even including mythical darkrai and arceus in the potential raid pool changes nothing about the types well expect to be best counters for the rest of gen 4 (shaymin, manaphy/phione are pokemon likely suited to spedial research, hence noninclusion) this is of course barring new moves being added/ move rebalance or community day moves. Essentially there's is no potentially better counter to any of the remaining sinnoh legendary Pokemon
Now the part I find interesting, Palkia is the only legendary who's counters had the largest potential for change, given Garchomp and dialga easily could have placed at top counters. Dialga being not weak to dragon makes it extremely reliable as a counter as well as having great dps an bulk, and garchomp being more or less a bulkier dragonite. While against the other 5 (or 7) theres very little in gen 4 that changes when it comes to the counter lists, the only one being potentially darkrai for the lake trio but it likely would be as displacing when it comes to the counters as say a dragon that isnt weak to dragon or another very high DPS bulky dragon to use against palkia, however much like mewtwo, their best counters will be moveset dependent> likely gengar for against swift, tyranitar/weavile/mewtwo (and potentially darkrai) for all (minus weavile v the one with fireblast)
*also note I didnt consider garchomp in for a potential palkia counter, even if it is released to day as it will be a very heavy investment to use with the candy needed to evolve, while great, it will be impractical compared to a raid caught palkia, had it ben released prior with more time to build candy, I feel it would be more practical
Might be a bit of overthinking on my part, but I like to be prepared. Just thought i'd share something I found.