This list presents the gifts in an extending or telescoping fashion. The ordinals modifying the list are not rankings of gifts per se, but a demonstration of how they arise in time during the development of a church. The counsel in v. 29 may seem to indicate that a qualitative discrimination is intended, but I don't think it fits the context. Would Paul have spent all that effort to illustrate gifts with the body analogy-- pointing out how needed each gift was, how much care and respect each one needed from the others, how necessary it was to be the gift one was intended to be, only to chuck it all with one verse at the end?
In other words, would Paul have said, "Be a toenail, we need toenails, your unavoidable destiny is to be a toenail, but desire to be a head!" I don't think so! Even though Paul does use the comparative (meizon) in v. 29, he did not do so to negate all that he had said from vs. 12-27. If so, what was he saying and why did he follow up his arguments with chapters 13 and 14? My reading is that the Corinthian church was completely out of order when it came to the practice of manifestations and spiritual gifts.
When they assembled, everyone was trying to one-up everyone else in speaking with other tongues. The spiritual development of the body was arrested, the telescope jammed, and the full scope of gifts was not arising and functioning as it should have. Everyone was stuck on what, really, was an initiation experience that everyone went through. Yes, it was a sign (manifestation), but it had no practical import corporately, other than to evidence that someone had been baptized in the Holy Spirit.
After the experience of the baptism in the Holy Spirit, the only good publicly speaking in tongues has is when it was combined with interpretation.
Interestingly enough, the fact that tongues is listed at all in v. 28 means that somewhere along the line in the development of the body, people will be gifted with an ongoing ministry of speaking (and interpreting) tongues. Granted, it will be when other body parts are more fully developed, but it is most certainly envisioned as a viable, body-blessing ministry. The rhetorical question of v. 29, then, is a self-evident corrective which reminds the Corinthians (and us) to practice everything according to its spiritual ontogeny.
