Largest set allowing one-step unstructured quantum search
Perhaps a more appropriate for your question: Dong Pyo Chi and Jinsoo Kim showed that for any "Grover-like" algorithm, in which we may change the phase of the diffusion operator and the oracle gate from $-1$ to possibly independent and arbitrary complex phases, a marked element can be found with a single query if and only if there are at least $N/4$ marked items. Here is a link to their article.

Note that the case $t=N/4$ was discovered earlier by Brassard, Boyer, Hoyer and Tapp.