Most powerful energy source you can only get by creatively cheating in a MFFS Admin Card, which gives you infinite Force Energy that you can convert into EU using the MFFS block i can't remember the name of. It runs at 2000+ EU/t too.
Oh, you're not cheating? Anything that has been mentioned in this thread. For the production of Biomass/Biofuel, the best source is still Saplings, and the best source of Saplings, not surprisingly, doesn't come from Forestry. It's a hard tie between Steve's Cart with Apple Oaks and ComputerCraft turtles chopping down Fir Wood.
ComputerCraft: + You get more saplings (If you set it up right)
+ Virtually no resources spent
+ Can be made Self Sustaining.
+ Consumes less bonemeal per wood/sapling
- Doesn't play well with chunk errors, unloaded chunks, etc...
- Only gives you wood and saplings
Steve's Cart: + Apples (Can be made into Apple Juice, which gives better rates to Fermenters & Carpenters)
+ Plays well with unloaded chunks and chunk errors. It simply resets and continues to run, virtually unbreakable.
+ Can be made self sustaining, but needs a Skeleton Spawner.
- Costs a fair amount of resources.
- The actual crafting takes a while. Close to one hour or more depending on your setup.
- Consumes more bonemeal per wood/sapling
- Doesn't accept Sulfur Goo as a fertilizing agent.
These are the two most efficient ways of producing saplings for fermenters to make into biomass/biofuel. I summarized the advantages and disadvantages i perceive, and i have to say, if you have the resources, i'd rather go for the Steve's Cart farm and not the CC farm. Simply because resetting the turtle after a server crash, chunk unloading, or other reason, can get really tiresome. The apples have a lot of uses, too. You can make short mead, you can use them as a liquid for the carpenter and fermenter, increasing their efficiency, and you can eat them. That being said, the ability to make a Sulfur Goo farm to supply your bonemeal is a rather nice addition. It's really easy to come by if you're quarrying near Extreme biomes, and can be smelted from gunpowder, too.
Biofuel can go into producing both MJ and EU now, so there's absolutely no need of setting up any other infrasctructure after you get into this point. Except if you want to.
FUN, but ineffective ways of generating energy would be...
MJ:
Blutricity Engines! Make a bunch of windmills around your base and set up a nice atmosphere. Really cool looking.
Set up a goal for yourself! "I'm going to generate 100MJ/t using ONLY Redstone Engines!!!"
EU:
MINE ALL THE TIN! Watermill energy! OVER NINE THOUSAND WATERMILLS!
Skybase, Storm Age + Windmills (Might work out really well, actually.)
Screw Compact Solars! OVER 9000 SOLAR PANELS!