Full Set (28)

This will look terrible witch the 29th slot from purchasing SotO ... haha.

Also, I'm a big fan of account-wide QoL improvements and willing to pay money for them, hence I have the full set of the shared slots, unlimit gathering tools, Copper-Fed Salavage Kit (the silver-fed isn't worth it, use a mystic kit instead), Lounge Pass(es), Material Storage Expansion up to 1K, etc.

Of course the value of account-wide upgrades increases with the number of characters you (actively) play on your account. To me the Shared Slots are a very high priority and I'd buy them before any cosmetic items or BL keys or equipment/build templates from the gem store.