Who is the most powerful being Yajirobe could beat?

Well?

Vegeta's tail

Raditz maybe.

I think a databook stated his power level was 1000 by the end of Z.

Weekly Jump stated Yajirobe's PL was 970, so Raditz would stomp him.

Yaj should be able to beat Goku and Piccolo from the Raditz arc in h2h, though... Until they fired blasts at him, obvs.

I read somewhere that Yajirobe's powerlevel was 2100. So he would beat the crap out of Raditz, along with all of the Saiyan saga heroes barring Piccolo and Goku.

That PL is from the Tree of Might movie pamphlet, which is pretty lulz worthy.

For example, it also has 1st form Freeza's PL at 85,000, Kiwi's PL at 8,700, Dodoria's PL at 15,000, etc... All of which are sharply contradicted by the anime/manga.

Suffice to say, it's not exactly a reliable source.