Britain, France, Germany, Austria-Hungary, Russia and possibly Italy.
Italy was involved in the Balkan crises of 1913, Macedonia, Albania Essad etc. but I'm not sure if they were classed as a major power.
True, they gave us a good spanking at Gallipoli and Kut.
Dunno if they were classed as a major power, although they did have an empire then. Been a while since I studied The Great War (shows) I tended more to focus on 1914-1918 rather than the preceding era.
Bismarck's diplomatic stategies of the late 19th century basically set up the allegiances of the beligerrants, Turkey was'nt included in that. It was foreign policy failings by the Allies (Britain and France) that placed Turkey with the Axis powers. If I remember correctly.
I think Turkey was a bit like "the elephant who was no longer in the room". Theoretically and historically a major player, it gave both Russia and Austria-Hungary an Eastern front to consider for many years. Inaction and decline in the Ottoman Empire meant that they could afford to get "picky" with each other withouot worrying about the Turks.