It is generally accepted that Gandalf was the wisest of the Istari, and possibly of the Maiar, which Cirdan perceived on his arrival. The question is more one of what is meant by wise.

Gandalf (olorin), had a healthy fear for the power of Sauron and morgoth, and wisely hid his movements and intentions from all the servants of the shadow. He had little ego, and no personal motivation, other than to fulfill his duty and return to valinor. This made him extremely powerful against the temptations and deceipts of the enemy, which Cirdan saw in him. This was why he chose him to carry the ring of fire, knowing that he was the least likely in middle earth to misuse it, while being powerful enough to gain an advantage from it. Gandalf from the beginning made use of his time searching for those like him, people of contentment, who desired only to be allowed to continue to live their lives.

Radagast ignored the peoples of middle earth amost entirely, and Saruman followed the trend of men, which was to find and ally himself with more powerful groups, coming to lead them and to make himself a power, although at first this was so he could face Sauron and defeat him for the right reasons.