Arjun has raised a serious question. To give a clear decision immediately was not easy. But the answer in any case has to be given. The question is related to followers/seekers of two established paths of devotion. In His answer, the Lord has indicated that those who worship the manifested form with qualities are the best knowers of Yoga. The answer contains deep indications. Just simple/mere devotion is not meant. Not mere show of devotion. Devotion is not just a matter of mere physical action but it is a means of seeking union with the Supreme. The devotion should be done in this form keeping in view this goal. By saying Te Mein yuktatatma matah the Gita's sermonizer has given a clear indication that who do devotion as mere outward show are not the best, but the seeker who seeks to use devotion as a means of becoming united with the Lord as a goal is in my opinion the best. Just not only this, examine the answer given by the Lord in greater depth:

The decision is of the highest court. Neither side should become arrogant. It should not be taken for granted that the decision is in our favour; we are the best and others are inferior to us. For such arrogant mental states the Lord has said initially Mayyavesh mano ye mam those who have fixed their minds on me, attachment-hatred, arrogance, etc. arise from the mind, remain/stay in the mind and once the mind has become one with the Lord then there is no possibility of their existence. Along with the concentration of mind it is also specified Nitya yukta Upasate. If for some time one meditates on the Lord and then again goes back to world this will not do. Then the seeker is not the best yogi in the eyes of the Lord. In the mind, there should be of constant strong relationship with the Lord. Wherever one is, one should be His. Whatever is done should be done as a worship of the Lord and as an offering to Him. Your mental feelings/ emotions should not get mixed up with the physical functions of the body. In emotions and thoughts not me and mine but you and yours.

Side by side remember this-Shardhya Parupeta, having, the quality of supreme reverence with faith. May there be complete reverence! My peace, my welfare is in devotion to the Divine, singing songs in praise of the Divine and attaining the Divine; there is no other means. Such a strong determination with reverence and faith is shraddha. This shraddha is also in every move continuous and undivided and always shraddha. Therefore along with shraddha the word Parupeta is used. Shraddha should be better than best.

Actually through the medium of this answer the Krishna of Gita has expounded form/character of real devotion. He has encouraged as well as warned the devotee. If you create such state/situation endowed with yoga you will excel as a Yogi.

The devotee of form with qualities is the best yogi, then what is the state of those devotees of Formless without qualities? Is it not a one-sided decision? The speaker of the Gita makes it clear in the following verses.