This is for competitive battling
Genesect faces competition it faces with better Steel-types. Its nearly unparalleled movepool and ability to run multiple offensive sets are not sufficient to make Genesect worth using due to its unreliability as a check to Xerneas and inconsistent damage output that is reliant on Download. Genesect requires its team to have extra security against Xerneas as well as other common threats like Primal Groudon and Ho-Oh that can pivot into Genesect. Burdening the team to deal with these threats further diminishes Genesect's viability as an offensive Pokemon and decreases the number of sets that Genesect can opt to use to benefit its teammates in return. As a pivot, Genesect can force switches quite well and thus function as an offensive pivot, but it lacks much damage output against defensive teams without a beneficial Download boost and a boosting item despite the multiple coverage moves it can run, while running dedicated sets that aim to wallbreak will make it helpless against offensive teams due to its middling Speed and bulk. Even a pivoting set using Choice Band faces competition with other pivots like Mega Scizor and Magearna. Mega Scizor has a much better chance to take on Geomancy Xerneas and its access to recovery enables it to switch into support Arceus formes as well, while Magearna, though it may suffer from its inability to pivot against Ground-types, is one of the best checks to Xerneas and checks other dangerous Pokemon like Yveltal. So, Genesect is pretty bad.
Rayquaza is very threatening against bulky offense and defensive teams with its great offensive presence, Flying-type STAB moves, and ability to be either a sweeper or mixed wallbreakers such as Zygarde and Mega Sableye and Steel-types such as Necrozma-DM. Defog users that fit on offense such as Xerneas and Yveltal alleviate Rayquaza's Stealth Rock weakness, though Stealth Rock is still a problem for Rayquaza over the course of a match.
Sources: https://www.smogon.com/dex/sm/pokemon/rayquaza/uber/ and https://www.smogon.com/dex/sm/pokemon/genesect/uber/ TL;DR: choose Rayquaza, it's got Dragon Dance and a gigantic movepool. Genesect is a inconsistent and unreliable Uber that is completely outlclassed by Magearna and Necrozma-DM. However, honestly Rayquaza isn't as good as other Ubers.
