Hard to pick one.
Pigmy has less damage potential, but she's more consistent. While weakness multiplier is better that Pygmy's buff, 45% is still... well, 45%. That's a lot. And extra crit for everyone is always nice, since RR is not the only one dealing damage in this team.
Fennel can really pump these numbers with 1.5 multiplier applied after crit damage, but on whaleship crystal/Nidhogg/other type-less bosses she pretty much becomes an overstatted 5*.