What pokemon(s) for RC parking?

I'd say not the mewtwo, you have a lot of candy there as is, able to add a second move to two or maxed another one out without dipping into RC, I feel you're fine there.

For what to store them in, either of the weather trio, groudon kyogre and ray are all good, groudon wont be too useful for raids for a while (dialga is handled much better by machamp) and heatran just left, we dont know when/if precipice blades may come out, or origin pulse for kyogre, who also doesnt have many legendary match ups, but it very dominant agaisnt ther 4's and some 1-3's (though as you said you dont have Good IV specimens of them). If you have any rayquaza, I recommend those most, we are likely to see it come back soon and it will be very good againt palkia, who itself will be a very good legendary dragon (according to pokebattler, actually a better counter to itself than rayquaza depending on palkia's raid moveset).

However, if you have a large number of legendaries, i's actually wait until tomorrow to decide what to do, raids change at 4pm EST and we'll get a new raid boss rotation and new tier 5 legendary, I recommend waiting for this most of all as theres no worse feeling than dropping a bunch of candies on a legendary to store them only to be unable to properly counter a legendary (only time I ever RC stored was in a ray right before giratina, accidentally'd myself into dodging a bullet bigtime)

Other than that, not much else I can recommend

I haven't RC parked before (+600 sitting in bag space) so my opinion may not be as sound as some others.

Depending on how many Mewtwo you have it may still be a solid choice. This goes double if you have multiple Shadow Ball Mewtwo. For raids, Shadow Ball has been its biggest claim to fame and it's no slouch in PvP either. Despite some naysayers claiming it lost some ground with the effectiveness rebalance it's still among the fastest counters to anything weak to ghost. You can pair a second attack on it for a variety of specialist uses or for an extra layer of unpredictability in PvP. Thunderbolt in particular has a very low energy cost, allowing you to force shields or deal large amounts of damage in relatively no time.

If you don't think you'll get a good IV version of Groudon or Kyogre or if you're worried about the potential onset of Precipice Blades or Origin Pulse I'd also consider them for candy banks. Even without their signature moves they're absolute units. Kyogre sees a lot more raid prevalence than Groudon while I see Groudon in PvP much more.

Groudon and Kyogre go away tomorrow, it may be advantageous to see who comes next and sink your candies accordingly. Otherwise consider some options for PvP if you play it regularly. Giratina-A is a killer in Ultra League, Heatran apparently does well in Master if you can clear out any Groudon first. Lugia has always been known as a wall, though when I squared up against one my Mewtwo was able to pick it apart pretty fast.

Kyogre and Groudon I think are the best for a very long time RC park. They have potentially both Primal forms and great exclusive moves in the future, putting them far above any competition in their types. Doesn't matter if you don't have good specimens now if you consider these two.

For more near future usefulness, I'd consider Rayquaza the best. It'll be the top DPS against Palkia and all 3 of Gen 5s legendary dragons. Mewtwo is also good for shorter term but not that necessary if you have enough Gengars, Weaviles and Tyranitars.