The stability of $+1$ oxidation state increases in the sequence:

In Group 13 elements ($\text{Al, Ga, In, Tl}$), the stability of the lower oxidation state ($+1$) increases down the group while the stability of the higher oxidation state ($+3$) decreases. This is due to the Inert Pair Effect (the reluctance of $s$ -electrons to participate in bonding). Thus, ${\text{Tl}}^1$ is the most stable.