None of the Vanilla saplings really have a huge amount of Sappiness. The Blue Mahoe has the highest sappiness in it's saplings, but it is a very slow growing tree. You could either use a cross-bred Blue Mahoe (I'd suggest with a balsa initally, to try get highest saplings and sappiness). Or the White Willow is the best for it without using hybrid trees, if I remeber correctly. I'm not sure which biome inspecific tree would be best purebred.
On another note, if you have plenty of apples (or lemons, or papyas) you could use a squeezer to make apple juice and mulch to fuel your fermenter and get a little bit of extra biomass.