I'd say a bunch of them are equal in this regard. It's hard to split Goku and Tien. Tien backs out of conflicts later on in the series, but only because he's developed a realistic view on his power level compared to the others. He doesn't develop this (in Tien's case, rational) self-doubt over his relative power very easily and it took many years of compiling evidence of others being stronger than him - for comparison's sake, someone who develops this same self-doubt easily is Krillin, who is quick to accept that others are stronger than him, with just a little evidence (I'd say this is because Krillin has lower confidence. It's not that he's able to get accurate reads more quickly. But I'd say Tien also had overconfidence when he was first introduced and had a life of being a frontrunner until the King Piccolo Saga, so it was hard for him to believe others could be stronger than him).
Vegeta seems the most confident though, particularly on Namek. Is that truly confidence though? Is that the best thing to call it? Idk. But he's delusionally overconfident. Whereas Tien was able to accept not being the strongest, Vegeta never accepted it as even being a possibility, even in the moments where it was clear to an objective outsider that others were ahead of him (eg vs A18 or Freeza) or in situations where it was logical to imagine he could be in danger (allowing Cell to become Perfect). However, it should be noted that unlike Tien, Vegeta has rational reason to believe himself capable of catching up to others, since he's a saiyan, has always been able to become stronger and sees another saiyan (Goku) do it.
On the other hand, Vegeta isn't that confident in the sense that he only sees himself as having worth if he's the strongest or at least is the strongest saiyan, which is why he gets so mad when Goku is stronger than him. So while he is highly confident in his power, fighting abilty and his ability to get stronger, he doesn't have the confidence in the idea that he has worth regardless of where he ranks in terms of fighting power.
Freeza is very confident, but his confidence stems from being the strongest around. It's pretty easy to be confident in that situation. Vegeta has his confidence even when he's experienced being relatively weak (though, he still knew he was the strongest of the saiyans for around two decades of his early life). He even retains the belief he can be the strongest after getting beaten down and mocked by the Ginyu Force, Freeza and then Android 18 (and to some extent by Piccolo. Mocked, not beaten down). So Vegeta's confidence has more evidence of resiliency than what we saw of Freeza's - but we never actually got to see Freeza be in so many of the confidence-challenging situations Vegeta was in.