What is his best move for DIing Fair or Dair? Also how good is lucario's glidetoss?
If you mean to cancel air momentum, Fair is the best move to use because it has the shortest duration time out of all of Lucario's aerial moves.
I'll assume that you know how to glide toss so I'll give you the numbers. The higher the numbers, the further he'll go (5 is the best.):
Lucario - 1.2051
F - 0.714
B- 1.6426
FR - 0.714
BR - 1.7497
In this situation, Lucario would be facing the left. (Works when facing the right too.)
F = Forward (Lucario rolls in the direction he is facing (Left) and the item is thrown in the same direction. So both Lucario and the item go to the left.)
B = Backward (Lucario rolls backwards (Goes right), but the item is thrown to the left. So Lucario rolls to the right while he was initially facing left doing a backwards roll while the item is thrown to the left.)
FR = Forward Reverse (Lucario rolls forwards (Faces the left, rolls to the left), but the item is thrown to the right/backwards. So Lucario rolls to the left while the item is thrown to the right. If done correctly, Lucario should face the opposite direction as he slides to the left.)
BR = Backwards Reverse (Lucario rolls backwards (Faces the left, rolls to the right) and the item is thrown to the right. So Lucario rolls to the right while he was initially facing left doing a backwards roll while the item is thrown to the right. If done correctly, Lucario should face the opposite direction as he slides to the right.)