I voted Reboot, it has the most awesome MechWarrior flair in my opinion (especially the computer voice saying that monotonous "Eject. Eject. Eject. Eject" at the end gives me chills for some reason :p). But generally I would rank the intros as followed:
Reboot
MW2
MW2 - GBL
MW3
MW2 - Mercs
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MC1
MW4
MC2
MW4 - BK
MW4 - Mercs
The MW4 intro is too cheesy. So is the MW4 - BK intro, but at least it's supposed to be cheesy, but the in-game sequences and all that is rather boring and ugly. There is a clear gap between the MW2++, MW3 and the MC1, MC2, MW4++ intros. The former are epic, the latter all pretty cheesy and often stupid. I am often wondering "what were they thinking".