Inheritance tax
I'd be interested to see data for how many people avoid inheritance tax. It's a very weird tax that seems to mostly catch those who die younger than expected. My suspicion is if the avoidance is high then it'd be better to replace it with a lower (as a percentage) wealth tax that with no effect on tax revenue. (I forget the difference between avoidance and evasion, which itself seems a very silly distinction of wording as avoid and evade are often synonyms)
ISAs
ISAs, like inheritance tax, also feels like government-"sponsored" tax avoidance. The large increases in the allowance to ~£20k by the Conservative government has made this a fairly easy way to avoid tax. Add to that that some people have more than 1 million pounds in these tax-free wrappers then it feels quite silly and in need of reform.
International tax
The wealth of companies/individuals is a function of the size of the world and this to me seems very unjust... The richest people essentially earn a constant times the number of consumers. If we happened to live in a world half as large, they'd be relatively half as wealthy as generally wealth is not so directly a function of the population.
International corporations choose where they pay tax. This feels very important and yet is seemingly impossible to solve, but I can only imagine that every year it is a yet harder problem as corporations become more powerful.