Overview:
Altaria has some stiff competition from its fellow Dragon-types and sadly is one of the lesser of the mighty hoard of Dragons. This does not mean Altaria is useless, however. Altaria is a more defensive Dragon, with 75/90/105 defenses being quite good and a Dragon/Flying typing that gives it 5 resistances and one immunity. It also has Natural Cure and Roost to make it an even better defensive wall. However, Altaria suffers from lower 70/70/80 offenses, which means outside of Dragon Dance and Hone Claws, it can do little back. Also, it's typing leaves a nasty Stealth Rock weakness and a double weakness to Ice. Still, as all Dragon-types have shown us, you cannot underestimate their awesome might, even if this particular Dragon's might is as a wall.
Abilities:
Natural Cure: Altaria is cured of major status conditions (Sleep, Poison, etc.) when it switches out. Awesome ability that works well with Altaria playing a defensive role.
Cloud Nine: Weather effects are negated when Altaria is out on the field. A good ability in any tier with weather, but unless Altaria is in Ubers or OU, Natural Cure usually is the better choice. Good to check rain teams, though.
Movesets:
Defending Dragon
-Dragon Pulse
-Toxic
-Roost
-Cotton Guard
Item: Leftovers
Ability: Natural Cure
Nature: Calm (+Sp. Defense, -Attack)
EVs: 252 HP, 4 Defense, 252 Sp. Defense
This is a basic outline for a tank, though Altaria makes it work a bit differently. Dragon Pulse has awesome coverage and hits decently hard (for no investment). Toxic is the standard move on many walls to wear out the opposition with Toxic Poison. Roost heals Altaria for 1/2 of its max HP and also makes it lose its Flying-type for the turn, which is good if the slower opponent tries an Ice-type move or something. Cotton Guard raises Altaria's Defense by three stages, making it really tough to take out. Leftovers are for HP recovery, Calm maxes out Sp. Defense, and EVs maximize special bulk.
Because Steel-types wall this set hardcore, an alternate Careful nature (+Sp. Defense, -Sp. Attack) with Dragon Claw over Dragon Pulse and Bulldoze over Cotton Guard can be used. If you have Steel coverage elsewhere, however, the first set works just fine.
Sweeping Cottonwing
-Dragon Dance
-Dragon Claw/Outrage
-Earthquake
-Roost
Item: Life Orb
Ability: Natural Cure
Nature: Jolly (+Speed, -Sp. Attack)
EVs: 4 HP, 252 Attack, 252 Speed
This set turns Altaria into a sweeper that has natural bulk. Dragon Dance boosts Altaria's Attack and Speed by one stage each, which makes Altaria quite deadly after enough boosts. Dragon Claw and Outrage are both awesomely powerful moves with excellent coverage. Dragon Claw is the safer choice that doesn't lock you into a move for 2-3 turns, but Outrage has a lot more power behind it. Earthquake basically gives Altaria perfect neutral coverage and is powerful to start. Roost helps Altaria stay alive and helps fight against Life Orb recoil and such. Speaking of which, Life Orb boosts the power of Altaria's attacking moves at the cost of some HP, and the nature and EVs maximize Speed and Attack for better sweeping capabilities.
Other Options:
-Heal Bell can be good for team support by healing major status conditions your other Pokemon might have, but since Altaria already has Natural Cure, it's not as useful for Altaria itself.
-Perish Song would be fun to stop opposing set-up sweepers, but Altaria runs into the four-moveslot-syndrome somewhat.
-Roar fits the same description as above.
-Hone Claws is another boosting move that does Attack and accuracy, but Dragon Dance is usually better.
-Sing could shut down an opposing Pokemon, but 55 accuracy just sucks.
Checks and Counters
Piloswine. Priority Ice Shard that gets same-type-attack-bonus (STAB) targeting Altaria's lower defense can and will shut Altaria down fast. Actually, any faster Ice-type Pokemon, and Ice-type moves in many cases, will really give Altaria a lot of issues. Stealth Rock is another way to get Altaria into KO range. Finally, Toxic and Burn can wear Altaria down if it can't afford to switch out or if Altaria has Cloud Nine for some reason.
Overall, Altaria is not my favorite of my dear Dragon-types, but one cannot say that Altaria is useless, as it has the right tools to do an excellent job.