Im not sure about what would i choose so few points about a few of the options.
Now, even though I prefer SPARTAN IIs in stories about Halo universe and the S-IIIs too when compared to S-IVs, I still think that I would much prefer to be a volunteer veteran soldier for the augmentations when compared to being a kidnapped kid raised for war or even one that would be from a program primarily consisting of war orphans.
Then about the ODSTs and Marines. Being a Marine would possibly be a good start but it wouldnt make sense in a long run considering their equipment is rather technologically behind when compared to what they are usually against. And the armor is not that good against the flood either, even if the flood is more of a biological threat rather than technological.
On the other hand, the ODSTs sure have rather useful pieces of technology at their disposal but they are still lacking in defense and they are, for some missions, send to rather hot situations. Though the stealth missions would be much more preferable than the usual Marine missions. Something like infiltration, sabotaging, quietly eliminating HVTs and such. Still, I think I would prefer SPARTAN branch to these ones mostly to even the odds with known opponents by getting access to more suitable gear for usual battle situations.
Now ONI is something to think about. Sure its secrecy would mean stealth oriented jobs but its morales are rather questionable. Id rather leave you guessing about this one.
Now lastly the navel officer, definitely no. But if you in any case happened to mean a naval officer then it could be something to consider. It could mean a chance for tactically shaking the opponents. Also it could mean going against more technologically advanced fleets but it also is where the battles mean most,and depending on the ship(s)/station(s) that is/are commanded the situation could go from manageable to suicidal. So perhaps because of a chance to really make a difference.