I'd have to go with the 5th campaign. The reason being is that although most of Shu's best generals were gone, Wang Ping, Ma Dai, and others were able to hold back Wei. And in 234, Wei had few good generals at the time, as their best ones left had all recently been killed. This one defeinitely has the most potential. The 1st one would be a close second for all the reasons listed.
Huang Zhong: Are you calling me old?!
Wei Yan: ...Yes.
Huang Zhong: Liu Bei! I demand you kill him!
Liu Bei: Well it's... true...