Latias

Latias is able to serve either as a late-game sweeper or as a check to an array of foes such as slower Dragons or physical attackers, with its better defensive stats, which allow it to take a Shadow Ball from Mega Gengar, and access to Healing Wish giving it a niche over the generally preferred Latios. Mega Latias's bulk emphasizes these differences even further, giving stall teams an immense amount of trouble once Tyranitar has been eliminated and making Latias more versatile than Latios. Furthermore, in combination with the recovery granted by Roost, Latias is able to stick around for extended periods of time, which resistances to common types and a Ground immunity help with. Its high base Speed allows it to outpace the majority of threats in the metagame, often giving it the initiative. This is particularly advantageous for outspeeding most Dragons, such as Garchomp and Mega Salamence, the latter of which often does not use a Speed-boosting nature. A wide offensive movepool, including moves such as Thunderbolt and Shadow Ball, gives Latias the means to deal with a variety of foes, making it somewhat unpredictable. For example, Azumarill, while ordinarily problematic, is 2HKOed by a Choice Specs-boosted Thunderbolt.

Latias does still, however, face competition from Latios offensively, as the latter can take advantage of a higher Special Attack stat in that role. Latios is also not the only Dragon-type it faces competition from. Hydreigon, for example, is another special attacking Dragon-type that hits hard. Being walled by both Dark- and Fairy-types can create some unfavorable match-ups straight from Team Preview. Steel-types also resist both of Latias's STAB moves, making foes such as Mega Mawile, Aegislash, Ferrothorn, and Scizor equally problematic. Overall, Latias is very versatile and can therefore perform well on many different types of teams, but you should ensure it's not outclassed in doing so.