Lugia for Great League

Lugia's a pretty good generalist with Dragon Tail/Sky Attack and has good bulk to top it off. Giving it hydro pump also makes it decent against a fair share of the great league meta.
It's claim to fame is having Dragon Tail and being a good generalist while not being a dragon, which makes it a very good anti-dragon mon, since sky attack hits dragons for neutral.

Lugia, like many of the top mons in the great league, does have glaring weaknesses to ice and electric attacks though. Against the top ice/electric attackers, Azumarill and Magneton (respectively), lugia has horrible match-ups. It also looses to umbreon & metagross, but that's not a make/break factor, especially considering how well lugia does against the rest of the great league meta.

Dragon Tail/Sky Attack lugia is probably pretty good for great league, but he needs hydro pump as a second attack to really shine, and that's a pretty hefty investment.

I guess to answer your question: Yeah, lugia should be pretty good in the great league, but is he worth the investment to you?

Cresselia's a better psychic pokemon for great league, due to psycho cut.

Dont know how good Lugia will do, mostly as its got a weakness to electric and ice. magneton is plentiful because of research encounter magnemite which can counter lugia pretty well,
theres also zapdos and articuno.

Lugia should be very good, it ranks high on neutral TDO. Sky Attack is a good charge move and you can probably win both Altaria and Cresselia with DT/SA. Having Hydro Pump for coverage would be great, but it's obviously a steep price to pay.

You should definitely try it and see how you like it. It's not Altaria and it's not Cresselia but maybe something like a mix of both. If you're ever having any trouble with Counter users, Lugia is the best wall you can bring against them.

Whenever you have a defensive pokemon with low attack (Lugia is the poster child for those), it's critical to get every drop of damage out of your attacks, and STAB is a must. So I'd stick with Future Sight and Sky Attack, both are very strong and have STAB.

Lugia is a solid choice for GL. Not so keen on HP for Lugia - HP hits the best counters for Neutral damage not SE (Electrics, Dark, Ghost - Rock is the exception to that).

Go for it, should be fun.

Edit - Checked the numbers - Dragon Tail generates the same energy as Extrasensory, and does more base damage. With STAB, they are almost exactly the same. So stick with Dragon Tail, but I'd still avoid HP unless you have a specific foe you need it to counter. That combination only has trouble with Steel (not unheard of in PvP, but HP doesn't make it a ton better.)