I think that the Australian and New Zealand ones are okay from a technical standpoint.
We have a lot of really bad state flags in the US. A lot of them originally involved slapping the state seal (not optimized for flag use, usually more-detailed) on usually some shade of blue.
That being said, a bunch of them have been revised. Utah ("The Beehive State") did a 2024 revision, which I think is a pretty strong improvement (and follows the "originally just the state-seal-on-blue" pattern):
to
And honestly, I think that most of those warrant a change more than most national flags.
However, if I were going to make changes to national flags...hmm. Well, there are a whole bunch of countries that have extremely similar flags. Nobody wants to back down on their flag, but honestly, the present state of affairs just kind of sucks from a functional standpoint, seeing as the purpose of flags is to let one easily identify something as being associated with a country. It'd be nice if they'd flip a coin or something and have one of the countries pick something more-distinctive.
One of the most-egregious is the flag of Chad:
And the flag of Romania:
There are a bunch more that look very similar at a distance or at small sizes or if one is being flown upside-down. I won't list them all here, but it'd be awesome if those would get sorted out.
Also, while the tricolor, as with the above flags, is associated with republicanism, and thus a lot of countries use it, and technically, it's nice (simple, easy to identify, no text), there are similar alternatives that would help make flags a lot more distinguishable. Take the flag of Czechia, for example:
Still three colors, but looks pretty different from most other national flags. You don't get into the "tricolor with very similar colors" problem.
