I've heard 'ŭaŭ', 'ho! ', and 'vaŭ'. However, I want to see if there can be a best, official translation.
Ŭaŭ is what I see the most, I also see mirinde as mentioned by Chuck Smith. I do not personally recall ever seeing vaŭ.
Ho! is more ambiguous, it can be good or bad.
However, I'm tempted to say it has a very slight connotation of pain or worry, as it's connected with "ho ve" and we see it in the title of "Ho, mia kor'".
When I look it up on vortaro.net, I find this:
*ho! Interj., esprimanta tre ĝenerale la vivecon de sento: ho, kia belegeco!; ho, neesprimebla feliĉo!; ho, miaj esperoj!; ho, kia malfeliĉo!; ho, terure!; «ho» ofte signas plendan staton, de l’ sorto malfacilan baton
So, it's clearly free to be used positively or negatively, but I think this definition supports the slight negative aspect. So if you're using "ho" where someone might also say "ŭaŭ", it might be a situation they find surprising and unpleasant. However in practical use, this nuance is probably not too influential.
Ho is also used when one adresses someone ("al Vi, ho potenca senkorpa Mistero!")