What spongeforge build can I use successfully with a 1.8.8 forge server?
None yet. 1.8.8 Forge is still in development and we haven’t made the jump yet. We will soon though.
Do you have an estimation? Like maybe within the next couple weeks, or month?
If you’re familiar with building SpongeForge from source, there is a 1.8.8 branch you can use.
SpongeCommon | SpongeForge
DISCLAIMER: The 1.8.8 branches may or may not work, and they might not have the latest changes available on the 1.8 download, so use at your own risk.