The reason "won't we" is the most suitable question tag for the sentence "Most of us will join the army" is because it reflects the expectation that the majority of the group will join the army, and the tag seeks confirmation or agreement from the listener. 

The other options are not as appropriate for this context:

"Isn't it" is typically used for statements where you're seeking agreement with a positive statement. For example, "It's a nice day, isn't it?"

"Aren't we" is used for questions that express doubt or uncertainty, which doesn't fit the context of the original sentence.

"Am I" is not relevant here because the question tag should refer to the group ("we") rather than an individual ("I").