This is a Smogon competitive only opinion.
Rayquaza is basically overpowered now that it has a Mega. Rayquaza has an amazing plethora of moves. Thunder, Fire Blast, Dragon Claw, Outrage and Earthquake among other moves are all at your disposal, along with boosting moves such as Dragon Dance. All this, coupled with AMAZING Attack stats and a decent Speed stat make Rayquaza the best killing machine out there.
But it doesn't end there. Rayquaza mega evolves using a move, namely Dragon Ascent. This allows it to hold an Item such as a Life Orb to boost the power of its moves.
Simply speaking, Rayquaza's new Mega evolution has increased its popularity amongst competitive battlers