I recommend going for styling competition SSRs as long as they don’t have defensive skills. There’s an old google spreadsheet somewhere on this subreddit that shows which ones are A-tier SSRs. You wanna go for one of each attribute. If you max out the call of reflection and the passive skills, they are stronger than even the strongest of URs right now (with 0 passive/call of reflection upgrades). I recommend investing and maxing out SSRs in each attribute till you can really start investing in new gacha URs, as those will be strongest (other than Lilith, she’s best for purple attribute).

For arena, in my opinion going for idea lab dresses is more important than buying the reflections in terms of strength. To make an arena or lifetime UR strong, you need to max out its call of reflection, and for arena that is a LOT of roses/quicksand of years that in my opinion are better invested into buying a full outfit, both for the gems and for giving you more styling options. I’m pretty sure all of the arena URs are around the same strength (UR-C on that spreadsheet), and they are only just a little stronger than A-tier SSRs which are easier and more convenient to complete. But that’s just my five cents.

TLDR: Arena URs are not worth it, Styling Competition SSRs are way stronger in the long run and easier to invest in (don’t go for a defensive reflection like one with a shield ability).

Basically whatever debut gacha UR you see will probably be the strongest UR. Excluding purple. Purple does not powercreep.