Proof of this? It's common knowledge on this site that Kyuubi>One tails. I'm pretty sure this is why.
hachibi told kurama that the other bijuus hate him because of his belief that all bijuus powers are deteermined by the number of tails they have. 5 tails,the ape,did far better than the other ones,and even sent hachibi flying IIRC.
Shukaku seems weak to everyone because he appeared in part 2,where all the power levels were weaker,look at hashirama then and now.
Gaara's dad compared gaara to the mini shukaku copy of gaara when he was a child,which he subdued. Shukaku controls sand and wind,his body when he was at clone stages are very strong,as he is a bijuu,it is safe to assume than his sand powers are much much higher than gaara's and his wnd techs are on anothe level as well.
Kyubi is the most powerful,that is all that is known,the rest is a toss up.
No way is that little 2 tailed cat stronger than shukaku.
Shukaku would just hit sanbi with a sand burial.