It feels to me like Latios needs to move up to A, where it in my opinion really belongs. Contrary to common belief, Latios is not outclassed by her sister as the fulfill very different roles. The set I will be looking at is the all-out attacker set.
Latios (M) @ Soul Dew
EVs: 252 Spd / 252 SAtk / 4 HP
- Draco Meteor
- Hidden Power [Fire]
- Grass Knot
This is generally the best IMO (thunder and surf works on rain teams though). The criteria for A-rank are as follows:
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time.
Now we must ask us, can the all-out attacker set sweep significant portions of the metagame? My answer is due to its amazing speed, this is certainly true. In S+A rank there is nothing I would call a counter to Latios. While this is true for many of the S+A rank as well, this just shows the exact reason why it belongs in that rank. In B-rank, for example, most offensive mons actually have a couple of "hard counters," Blaziken has Giratina, Kingdra has Ferrothorn and so on. Looking at Latios placement in the B-rank, I am a bit confused since the only thing that can claim to really hard counter it is Arceus-Steel in the rain, and too an extent Ferrothorn. How many pokes in S+A rank doesn't Latios 2HKO?
While Latias is indeed better at checking Kyogre, you can't really complain about Latios special defense, which is high thanks to soul dew.
252+ SpA Kyogre Water Spout vs. 4 HP / 0 SpD Soul Dew Latios: 92-108 (30.46 - 35.76%) -- 44.53% chance to 3HKO
And why try to counter the uncounterable king of Ubers when you can destroy it with offensive pressure?
The advantage Latios has over A-rank special sweepers like Mewtwo and Palkia, is that it has significantly higher special bulk, about 30 % higher than Palkia according to Superimp's A Defense to Reach the Stars
thread (unless I have missed the point about logaritmic scale bases completely).
The other thing it has is an immunity to spikes and toxic spikes+ a neutrality to stealth rocks. This and its typing grants it way more opportunities to switch in that for example Mewtwo or Rayquaza.
Did I mention any standard cores in the metagame? Well, no, but what needs to be said is that they are actually ripped apart. For example; a standard sun-core consisting of support Groudon, leftovers Palkia and Ho-oh (and maybe Forretress as well) falls flat to Latios if SR are up, and even if there are no rocks on the field, Ho-oh will be hard pressed to survive assaults from Latios because of psyshock.
Standard sand core- TTar, Ferrothorn and specially defensive Giratina is another core that has trouble with Latios.
252 SpA Soul Dew Latios Grass Knot (120 BP) vs. 252 HP / 180+ SpD Tyranitar in sand: 202-238 (50 - 58.91%) -- 71.88% chance to 2HKO
252 SpA Soul Dew Latios Hidden Power Fire vs. 252 HP / 208+ SpD Ferrothorn: 308-364 (87.5 - 103.4%) -- 25% chance to OHKO
252 SpA Soul Dew Latios Draco Meteor vs. 248 HP / 252+ SpD Giratina: 438-516 (87.07 - 102.58%) -- 18.75% chance to OHKO
A layer of spikes up, and this core is put under immense pressure, and will be forced to predict a lot to deal with Latios. Latios, on the other hand can spam Grass Knot on a predicted TTar switch-in, and if Ferrothorn shows up, it will go down. Same for Giratina, meaning Latios will usually take down two members if it is played correctly (which is not that prediction reliant compared to what the sand core will have to pull off).
I also believe speed is very important for a sweeper. While unboosted dragons such as Rayquaza, Kyurem-W and Reshiram are slow, Latios has an advantage in its blistering 110 base speed.
Now of course there are some disadvantages with Latios (some would at least say, I think there are pretty much none). While I do agree it has trouble breaking through steel types in the rain, it's not exactly like we are talking about big threats just coming in and getting a free set-up. Sure Ferrothorn can come in, but your team can most likely handle it. Same goes for Arceus-Steel, who is an amazing stall-breaker, but you can always use Ho-oh together with Latios for amazing offensive synergy.
Latios really deserved A-rank.