Latios-Mega, honestly, isn't worth the mega slot on your team. Latios with a Life Orb can handle most of what Mega Latios can without taking up a Mega slot on the team. However, there are a few reasons why you might use Mega Latios - its better bulk and higher Attack stat.
Mega Latios can run a few different sets more effectively, most notably its Dragon Dance set, which has a surprise factor in it. However, because of its lack of an item, it's not really too powerful. Its moves of Dragon Claw, Zen Headbutt, and Earthquake, however, does cover a decent portion of the OU metagame, bar stuff like Ferrothorn and Skarmory. Its special attack can be used by stuff such as Calm Mind, if you so desire. However, the immediate power that Life Orb provides is what Latios uses to muscle past various threats of the OU metagame.
td;lr: Mega Latios is subpar, you're better off using normal Latios and a different Mega. However, you can run boosting sets on it, as well as a mixed set with Earthquake.
- Magnezone is a good partner, taking care of Steel Pokemon by trapping them with Magnet Pull.
- Similarly, Pokemon to deal with Steel Pokemon, such as Fighting types, are appreciated. Fighting types also take on the fat blob known as Chansey as well.
- Steel Pokemon: They can take out troublesome Fairy Pokemon that Latios can't muscle past. However, if a physical set is chosen, Fairies are less worriesome as most don't have the best Defense.
- Pokemon to deal with Pursuit: Latios often cannot touch Pokemon who run Pursuit, such as Metagross, Bisharp, and Tyranitar, unless Latios is running Earthquake. Earthquake needs prior damage to knock out any of them, however.
- Ferrothorn and Garchomp - Latios is extremely prone to priority, and both Pokemon, when equipped with Rocky Helmet, punish priority users.