Okay, here's the situation. I have 2 cyber dragons on the field and photon generator unit face-down. My opponent summons Mobius the Frost Monarch and i activate my photon generator unit when he summons it.

Would my photon generator unit come first or would his mobius' effect come first. He says mobius' effect would kick in first but i want to be sure.

It really doesnt matter. You chained the effect. So your card effect still resolves.

This is exactly like other situations (Mobius vs. MST, Mobius vs. Bottomless, etc).

Mobius is a mandatory activation as soon as it is Tribute Summoned, making it Chain Link 1 (unless something like Stumbling is in play, which automatically takes the first Chain Link) and the player who summoned it selects 0, 1 or 2 targets.

When the player passes priority to you to activate something, you can chain to Mobius's effect with, in this case, Photon Generator Unit, Tributing the 2 Cyber Dragon for the cost of activation.

Assuming nothing else is added to the chain, Photon Generator Unit, being Chain Link 2, will resolve FIRST, and Special Summon Cyber Laser Dragon. Mobius, being Chain Link 1, will resolve and destroy Photon Generator Unit (NOT "negate" or stop it in any way) and the other selected m/t (if there is one)

just to note, mobius is not a mandatory trigger effect, he is an optional trigger effect. when summoned you can choose to not even put its effect on the chain.

but other than that everythign else was right.

Thanks for your help guys.