Which Groudon should I power up?
Some of you may not believe I'm asking this question ... I did my first legendary raid, Suicune, just a month ago. But, amazingly I have since done 6 total and caught 1 Suicune and 3 Groudon (city players may laugh, but I live in a rural area ... although the raids were in the city. A discord invite made all the difference). So now I have a dilemma I never thought I would :D
I started to power up my first Groudon, caught at level 20. His IVs are good (12/13/14; 87%) and - never expecting to get more - I started to invest my very limited rare candy in him. But in the last three days I caught two more - one weather boosted - and now I don't know what to do. See the following screen shots ... which one should I invest in (I may never get another ;)
Also, what is the best moveset? I assume mud shot/solar beam, but do you agree?
Thanks for any advice!
1. CP 2690, powered up to 23.5
2. CP 2276, level 20
3. CP 2838, level 25

Answers
If rare candy is limited, do not even consider the L20. Are Fast TMs also in short supply for you? I would personally pick the L23.5 because it is the only wonder and unlike the L25, does not need any TMs.
Despite doing one Groudon raid per day, I have only caught five Groudons. My best two are 14A/15D/12S with DT/Solar and 14A/13D/15S with MS/FireB.
If rare candy isn't falling from the sky, I wouldn't take even your best one past 35 any time soon. I think having more useable legendaries are better than just one superstar. As you are building legendary depth go ahead and take some things to 30. Anything after 30...have a clear goal/focus about doing it.***I won't get into math, but I like when I get Solar Beam. I'm okay on potions and revives, but at some point I might not be...nice to have tanks with the powerful solar beam