A lot of endgame gear comes from Abyssea. Its not the only place to level 75+, but it is the most effective.
Its for solo, low man, and full alliances, it really depends on what you are doing. You probably won't do much soloing till you can get a hold of abyssites/atmas.
I would suggest working on abyssites/atmas first.

Go for abyssites of sojourn and celerity to allow you more time inside of Abyssea.
Go for abyssites of furtherance and merit to increase your stats.

Key atmas you need to get are Atma of the Minikin Monstrosity and Beyond if your a mage. Razed Ruins and Voracious Violet if your a melee DD.
You can use up to 3 atmas for your character, but in order to do that, you need to obtain 3 Lunar Abyssites by killing the bosses of the zones.

Definitely look up abyssites and atmas for a better understanding:
http://wiki.ffxiclopedia.org/wiki/Abyssite
http://wiki.ffxiclopedia.org/wiki/Atma