I think it’s somewhat hard to rank them without regard to team and playstyle.
Skadi is probably by far the best–IF you can quick loop. This is because her buffs are single-servant (both charge and quick buff), while Waver does teamwide for 20% NP, one servant for 30%, and teamwide ATK buff. So, if you can’t loop at least once, to me she’s not as useful by herself if you need to bring 2 servants to do the job for successive waves. Also, the general ATK buff she gives is actually a DEF debuff, which can be resisted as well as not persisting across waves.
Likewise, Tamamo is tuned for her niche–Arts looping/NP spam with as much stall as you want to play with. Given that her almost perfect partner is Waver, the two of them pack quite a bit of stall even in an offensive-oriented team.
But Merlin I think is, like Waver, far more general. Even though he has that Buster buff, it’s also tied to a nice 3 turn HP increase that I’ve used for that purpose a lot. Even in a buster team, I often pop that on T1 to give some squishy servant some breathing room before his heals kick in. Yes, he’s not a 3T support, but in the longer fights, the healing sustain as well as NP charging over time (+25% NP teamwide over 5 turns, and can overlap with multiple NPs) makes him quite useful in longer, tougher fights.
Luckily, I myself do not have to choose an order, since I’ve gotten the 3 available already, having been playing long enough to do so. And I have 1200 SQ ready for Skadi. Like I said, if you have the Quick servants that enable looping, it’s pretty much a no-brainer. But if not, then she’s more equal or maybe even not as general to you than a Waver. I don’t have any NP > 1 quick (SR/SSR) servants, so my ability to loop will not be great. So she might only be situationally used, maybe even only using plugsuit with Waver and/or Merlin (and/or a support Skadi).