Among the given options, Americium ($\text{Am}$) shows the maximum oxidation state. While lanthanoids like Lanthanum ($\text{La}$), Gadolinium ($\text{Gd}$), and Europium ($\text{Eu}$) predominantly exhibit a $+3$ oxidation state (with $\text{Eu}$ also showing $+2$), actinoids exhibit a larger range of oxidation states due to the comparable energies of $5f$, $6d$, and $7s$ levels. Americium ($\text{Am}$) can show $+3, +4, +5,$ and $+6$ oxidation states, making it the one with the maximum oxidation state among the choices.