My favourite ORAS Mega Evolution was probably Mega
Flygon Swampert or Altaria. Swampert is one of my favourite starters ever (behind Venusaur) and so I was psyched when it was revealed to have a Mega Evolution. Whilst I do like Sceptile's sleek design and cool typing Swampert retains what it already had - bulk and power - and just emphasized it through a bulkier, more muscular design. Swift Swim was a nice ability for it to fit on Rain teams competitively, but maybe it would have preferred an ability that focused more on it's Attack or bulk, but regardless it's pretty nifty and I enjoyed abusing it on a Rain team in early ORAS against people testing the OP Mega Salamence in OU before it got banned.
Altaria become a ton more adorable being surrounded by fluff and it got an incredible typing, much more bulk and a sweet ability in Pixilate. I was never able to successfully build around it in ORAS but it was a perfect bulky sweeper with Dragon Dance or a solid pivot with it's bulk and access to utility like Heal Bell and Roost. I was particularly looking forward to using it on one of my ORAS in-game teams, and my friend told me when he used it that Altaria performed well. An all-round decent Mega Evolution.