Triple Shot Espresso.
The 5,000G you spend on the recipe is totally, 100% worth it. Who wants to be drinking coffee seven times a day? Instead, scull down some Espresso at 6am, 2pm, and 10pm and you're good for the Day. And the Night, I suppose...
Otherwise I'd have to say the Galaxy Hammer. First you have to give up the pinnacle of all things shiny for a Galaxy Sword, and then you have to pay 75,000G at the Adventurer's Guild. Which requires said Guild to be open on arrival. Which it never is. So, definitely big-ticket. But once you have the Hammer, you'll be the boss.
Slay three slimy Serpents in one super slam? Sure.
Destroy so many darned Dust Sprites that the game slows to a dangerously delayed rate? Done.
And last, but in no, nil, nada way last is the Return Scepter.
I probably use it at least seven times a day at least. In fact, I go to bed cradling it in my arms, so that once I've checked the TV and Calendar –
PPNNEEUUGGHH!
– I'm off to my front steps, so I walk back into my house say hello to Maru and give my hat stands a pet –
PPNNEEUUGGHH!
– I'm back on my porch.
Is it more efficient? Maybe slightly.
Is it noisy? Definitely. So noisy Marnie's probably wondering what's wrong with her microwave.