Nero Claudius Drusus. It's not even a contest, since he was so damned capable, Augustus liked and trusted him so much, and his OTL death was just so very, very dumb. His survival as a POD also prevents OTL's too hasty abortion of the Germanic expedition he was leading, which would in all likelihood leave affairs on the northernmost border in better order. (None of that "and then he magically annexed lots of Germania" nonsense, but he'd secure a set of stable and far-more-loyal-than-in-OTL Germanic client states across the Rhine. That would make for a nice buffer, and potentially a future springboard for very gradual advances deeper into Germania.)
A key issue here is that Tiberius was the older brother, and also got along well with Augustus. When granting the two brothers honours, it is attested that augustus favoured Drusus. On the other hand, after Agrippa died in 12 BC in OTL, it was Tiberius who was ordered to divorce his wife (Vipsania Agrippina) and marry Agrippa's widow-- Augustus' daughter Julia. This rather indicates that Augustus was going to make Tiberius his heir even before Drusis died in 9 BC. Counter-argument: Drusus was married Antonia Minor, and that tie was considered an unbreakable one, whereas Vipsania was "merely" the daughter of the late Agrippa (from his first marriage). Can we say that if Drusus had lived on Augustus would have named him his heir, instead of Tiberius? Well, not would have, but certainly could have. That's good enough for this particular discussion thread. (Alternatively, we may switch the fates of the brothers, and imagine that tiberius falls off a horse in 9 BC, instead of Drusus. Problem solved.)
Having Nero Claudius Drusus as heir means having Germanicus as his heir (so this is a two-for-one, sice he's listed, too!), and we may assume that simple causality will prevent the circumstances of his OTL death in 19 AD. All of this keeps that bastard Sajanus nicely side-lined, which means the two elder sons of Germanicus (Nero Julius Caesar Germanicus and Drusus Julius Caesar Germanicus) don't die in 31 and 33 AD due to that one's perfidious influence. That should nicely keep Caligula out of the succession, at least.
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Second best option: Marcus Vipsanius Agrippa. Tiberius and Drusus were only considered because Agrippa kicked the bucket at age 51. To the best of our knowledge, due to natural causes. Let's assume those are preventable causes (like random disease) rather than something more inevitable (like a heart condition or something). Agrippa survives. We may assume he'd be quite capable, although I still think Drusus would be better (if slightly, and in part based on the positive side effects his survival would have regarding Germania). Agrippa's heir would be either Gaius Caesar or Lucius Caesar, assuming that their father's survival butterflies away their own early deaths. As far as we know, they both died of illness. Do note that both were not known to be stellar leaders. A bit mediocre, really. This might be their youth and inexperience, but when one compares either to Germanicus (who would be heir is drusus succeeds Augustus), they can't really hold a candle to him. So for multiple reasons, I consider Drusus a much better heir than Agrippa.
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Third best option: Tiberius, as in OTL, except Sejanus gets trampled by runaway goats or something.
Failing that escape clause, third best would instead be Marcus Claudius Marcellus, the actual heir by blood, closest male relative in the line of succession. If he survives, one may suppose it's going to be him. Since he was also married to the daughter of Augustus, this would yield the advantage (at least, some call it that) of establishing firm dynastic inheritance, and avoiding the precedent of adoption. Presumably, this would cause the Roman Empire to be much more dynastic in its succession approach, which some have argued would lend itself to greater stability (since it's always clear what the exact line of succession is). That and side-lining Sejanus would be the only advantages, though. I don't get the impression that Marcellus was a particularly exceptional leader, unlike the aforementioned candidates.