Ok, so I was playing Light and Darkness dragon, with Formula Synchron, Junk gardna, and Armory Arm out on the opponents field. Then the opponent tries to use eff. veiler on Ladd, and Im telling him that its eff. was negated. Whos effect was negated?

Well I am sure LaDD has no problem negating Veiler

One more thing to note, Veiler can't negate it's revival effect

Veiler's effect gets negated before it can resolve. All it does is target Light and Darkness Dragon.

You're both wrong -.-
It depends.

Chain links 3 or higher resolve backwards.

So...
Chain Link 1: Spell trap/whatever
Chain Link 2: LADD
Chain Link 3: Veiler

Veiler's effect would resolve.
Then again, this is only useful if you accidently waste your game winning card by using it's effect. When LADD's effect is negated by veiler it stops the chain and returns LADD's atk and def to normal. It can then later be used on the same turn.

Any Questions?

actually everyone who replied is right... it depends on the situation and you didn't give us much info to answer your question. Was he using veiler as chain link 1? or were you using Ladd to negate something else already?

They key point is, Ladd CAN negate Veiler, but only if Ladd hasn't used his effect already within the chain. Ladd's effect can only occur in a chain one time, and until that chain resolves, it can't negate anything else.

So if Veiler is chain link 1, Ladd can negate Veiler as chain link 2. If Ladd is chain link 2 negating something else, then veiler can negate Ladd as chain link 3, and Ladd CANNOT activate again as chain link 4 to negate veiler at this point.

I don't see him saying Veiler was chained to LADD's effect.

But either way I'll just sum it up with "LADD can ONLY be used ONCE per chain".
Should be pretty easy to figure out your answer by yourself, by knowing that.