^ Well, they're all pretty notable in-universe, I'd say.
Darrians have a reputation as the most technologically advanced culture in Charted Space (how true that claim is tends to be a complicated matter...). They're also the Imperium's best ally Behind the Claw. I imagine much of the Imperium knows about them, albeit with fanciful ideas about fleets of floating cities and sun-busting missiles.
Geonee were one of the Vilani's only real rivals -- they're one of the oldest spacefaring human races -- and have been a part of the various Vilani empires since the Consolidation Wars, while clearly still wishing to assert themselves as an independent culture. Also, they're the Ancients, they say so, do not doubt them, all the evidence is on their side, respect your elders.
Bwaps were the most successful of the Vilani subject/client races, probably the only one that actually expanded its territory and influence under Vilani rule (they're basically the Vilani's henchman race, only flexible enough in their paradoxically rigid way to keep themselves going as a cultural force through all three Imperia). Everyone in the Imperium knows what a Bwap is.
If Bwaps are the Vilani's "henchman race", then Dolphins (and Orca) are the Solomani's "henchman race", arguably. Their best allies, being their creations, but Solomani aren't always the best parents. "I made you, you do what I say". They sort of embody the whole idea of uplift and the social and ethical complications that come with it, and the fact that they're the "third Terran race" after Humans and Vargr, but aren't Major Race status, is also interesting I think.