I agree that Castrum is the most efficient but only if you aren't into PVP and don't have a party of friends who can consistently run through Tam Tara super fast. Castrum is boring though. I don't like PVP myself, so I've been doing my daily main scenario roulette (taking my chances on getting Prae instead of Castrum I guess, but the exp bonus is good while I'm leving my WHM) and then doing Tam Tara in party finder and taking my chances on getting a speed running tank, which happens pretty often so it's been working out all right.

I don't think it needs to be a completely maximized efficiency grind though. I've been working on it some each day and have a decent amount of tomestones already, and we have more time still.