Which is better: praying in the back rows of the mosque, or in the front rows on the upper floor?

The Prophet (peace be upon him) commanded to complete the first row, then the one after it, and made the best rows for men the first one, due to their proximity to the Imam. Praying with the Imam on the same floor he is on, even if at its very end, is preferable to praying on another floor, even if at its very beginning. The issue of the worshipper being at a higher level than the Imam is a matter of scholarly disagreement, and many scholars consider it disliked if the Imam prays alone below while the rest of the congregation prays above him. However, if someone is with the Imam, there is no dislike. Praying on the lower floor is closer to the Imam, so it is preferable.