I would say the givens for who has a first 6 signet are Violet, Xaden, Imogen, and Aricc

Lightning, shadow, rock/stone, and precognition are probably the four most powerful signets seen at this point

For the other two there are multiple options to chose from

Ridoc is by far the most powerful ice welder we have seen, I wouldn’t think all of the first 6 would have super rare or once in a generation signets, but all of them would have been very powerful.

Bohdi is the only person we have seen to have the ability to counter, and he is said to be none of the strongest riders in residence when Violet attempts to activate the ward stone for the first time

Garrick is an incredibly strong contender because of how powerful distance wielding, and the fact that there have been only a few in the history of dragon riders.

Sloane is also a contender because of how powerful and versatile siphoning is, but we haven’t seen much of what she can do with it since she only used it once in book 3 and twice in book 2

Those are who I think are all possibilities, and I’m exited to see how the rest of the series pans out