This isn't a good question because stamping out corruption in an ally (of sorts) is generally desirable. Even more desirable if you fund a major part of the international financial bailout system, like the IMF. Corrupt countries tend to be money sinks.
Of course one can take the ultra-cynical view that keeping a country corrupt and dependent on IMF bailouts is somehow an advantage for the US giving them extra leverage. But that's probably not the mainstream viewpoint given all the downsides of corruption.
There's article on how corruption in Iraq for example is major problem for the US exerting influence over there, while it is a boon for Iran's influence.
Wikipedia has some stats for how widespread corruption is in Ukraine, but these mostly involve the view from the local population. As for the big-business view from outside, which would be most interesting to the US directly, EY (Ernst and Young) had a survey of
senior executives with responsibility for tackling fraud,
bribery and corruption. These individuals included chief
financial officers, chief compliance officers, heads of
internal audit and heads of legal departments. They are
ideally placed to provide insight into the impact that fraud
and corruption is having on business globally.
For Ukraine, 0% (yes zero) of the respondents said that they believe "authorities are willing to prosecute and [are] effective in securing convictions" on corruption cases. And 88% agree that corruption is widespread in business in Ukraine. Which basically means that they believe that doing business in Ukraine entails at least dealing with some of the competition engaging in corruption unimpeded. As you should know, laws in the US prohibit companies giving bribes abroad.
And it's difficult to point to any case in particular because no high-level convictions have been secured:
Poroshenko introduced an anti-corruption agency, but according to the non-government organisation Transparency International, it has failed to bring to account any corrupt high-level official.
So we'd have to resort to mere allegations for big cases.
Probably the highest profile case of corruption without individuals being found corrupt is that of the former president's Yanukovych [entourage]:
Prosecutors wrote in the indictment that Yanukovych headed a criminal organization, and named 56 members of the group that ultimately came to be known as The Family. The judicial decision [...] said the group laundered state assets through 400 companies, including Kurchenko's Prosperity Developments.
The $1.5 billion was laundered through foreign accounts, prosecutors said, then returned to state-run banks controlled by The Family as securities.
Despite obtaining reports on the missing billions and cooperation from foreign governments, Ukraine has not convicted anyone from the Yanukovych regime.
Yanukovych himself happened to be pro-Russia and rather anti-EU/West. An Ukrainian court did find him guilty of treason though, but apparently not because of any bribes taken or fraud, but just his policies, i.e. "complicity in Russian military intervention in Ukraine" as Wikipedia puts it.