Whilst studying the Julio-Claudian period I noticed that there seemed to be a lot of reference from my teacher and the text book to Germanicus being a great military leader. I believe Tacitus states that 'he would have equalled Alexander the Great'. I don't quite understand why Germanicus seemed to be held in such high regard, because the only reference to any of his military exploits I could find were to a kind of 'meh' Campaign against the Cherusci in order to find the remains of the three legions killed at Teutoberg. Certainly nothing Alexander the Great level, in terms of impact and military prowess. Were there some other campaigns that Germanicus was involved in that gave him this reputation? Or was his reputation a result of propaganda?
My personal opinion after reading lots and lots of Tacitus is basically in agreement with you that Germanicus was a fairly mediocre general. His Germanic campaigns were half successes if that, but he had a major advantage of timing in that they came immediately after the disaster at Teutoberg, and so even though it was only a half-success it could be bolstered as having "restored the glory" to Roman arms. I think in American history something like Antietam might be broadly comparable as really only a half success, but that was enough for the time. The fact that he died young only solidified his reputation.
That being said, probably the most important reason for his fame today is because of Tacitus himself, who very carefully sets Germanicus up as the "hero" of the early part of his narrative, both to serve as a foil for Tiberius and as part of his broader purpose of discussing the individual against the machine of the imperial system.
If Germanicus had marched against Tiberius and become Emperor, who knows what he could have done. Would he have been on par Alexander the Great? No... The comparison to Alexander mostly has to do with age. They both launched military careers in their 20's, and died at 32. Germanicus was most likely poisoned and Alexander could have been as well for all we know. Also both were never defeated in battle, although Germanicus did find himself in a stalemate during the Pannonian Revolt in which Tiberius sort of saved his ass, but he was still never defeated.
When compared to other Roman Generals, Germanicus is probably in the middle of things. He in no way was at the level of Julius Caesar or Scipio Africanus, but he did have a successful campaign against a relatively formidable enemy.
In reality much of the praise for Germanicus comes from the public's disdain for Tiberius.
But Germanicus got the last laugh by being the father of Caligula and grandfather of Nero.