You need to cover your fellow team members' Grass, Water and Ground Weaknesses.
Grass Weakness - Fire, Flying, Bug, Poison, Ice
Water Weakness - Grass, Electric
Ground Weakness - Grass, Water, Ice
Flygon can learn Fly and Giga Drain / Solar Beam, thus covering all.
Altaria can learn Sky Attack (till lvl 49, Fly (I'm pretty sure there would be a move deleter)). It has poor Attack, but has access to Dragon Dance to raise its attack, but it is almost useless as you need to use Special attacks to cover the weaknesses.
Salamence is pretty good in stats, but can't cover its fellow 'mons' weaknesses. (Only Flamethrower)
Dragonite has very good bulk, awesome Atk, good SpA and decent Speed. It can learn Surf/Waterfall (an extra Surfer for you!), Ice Beam, Flamethrower, Thunderbolt, Fly, Dragon Claw and many more! So, Dragonite is better among the Pokémons you have mentioned.
Dragonite @ (anything)
Nature: Catch it again if you get -SpA nature.
Ability: Inner Focus
- Fly (Grass types)
- Ice Beam (Grass types + Ground types)
- Thunderbolt (Water types)
- Surf / Waterfall (if you need extra surfer) or Iron Tail (You need to cover it's Ice and Rock weaknesses!)
Note: If your counters are Swampert and Quagsire, then continue with Flygon
Flygon @ (anything)
- Sunny Day (for Solar Beam)
- Crunch (filler)
- Solar Beam (Water and Ground types)
- Giga Drain (Healing, plus Water and Ground types)
You might want some Ethers or Elixirs to rise the PP (if it is there in GoD).
The reason why I mentioned Ground/Water/Grass is simply because most Water-types can learn Ice Beam, and since Grass/Ground would make my Electric moves a lot less effective, those multi-types are the biggest threat.
If that is the case, use Flygon, or any other Grass Type.
My current team w/o Lugia is :
Giga Drain / Solar Beam knocks Swampert / Quagsire out, as they are your biggest threats.
Your Ninetales should knock out Sceptile.
Use Lapras instead of Walrein, as it learns Ice Beam and Sheer Cold by level up. If you want the ability Thick Fat, then go with Walrein.
Also, If you want to include Lugia, you might want to replace it with Ninetales I suppose, but you would come with 3 Pokémons' weakness to Ice (Claydol, Flygon and Lugia). So you might want to keep Ninetales, but replace Claydol with Flygon.
Till Lugia gets Aeroblast, your Ninetales should handle the grass types. You might have Flamethrower for Ninetales (if you had evolved Vulpix earlier, then Fire Spin is the only move viable, except TMs or Egg moves. Or give Lugia TM - Aerial Ace (Super Effective to Ludicolo also).
So you might want to replace Claydol with Flygon (with the above moveset), and keep Ninetales. Also, Lugia would learn a Psychic type move by Level up, so you might not care about Grass / Poison types.
Hope this helps :)