[viet]

I'm not sure which nitrogen would most likely to protonate in this structure.

[Doc Oc]

All of those nitrogens are unique in their reactivity.  What are the key differences?

[viet]

the key difference i think would be the Nitrogen that has 2 methyl groups, would that be the one to protonate?

[Doc Oc]

You're on the right track, but just being bonded to methyl groups is not an indicator of anything.  The other nitrogens are also bonded to two carbons, but their reactivities are different.  There's more to it than that.

[lavoisier]

viet, you could also look at it this way: for a nitrogen atom to be basic, it must be able to bind a proton (H⁺), using its electron pair.
In which of these 3 nitrogen atoms is the electron pair more available to be used in such a bond? And why is it less so in the other two?

(This is just a reformulation of what J-bone was trying to tell you.)

[viet]

thanks