I guess it depends upon how you build you army (which is also makes the book great- so many good builds).
Astopath is probably my #2 choice, but I also don't build my army to depend upon my out flankers (ususally a penal legion or two and a sentinal squad or two)
Officer of the fleet is nice, but so very often I've rather have a medic in the squad - I had one entire touriment where this guys actually did me more harm then good - kept all the opponents with reserves off the table for too long in a KP game!
But mister ordinance has paid for himself so very ofter
for me its astropath first and officer (far away) second. mainr eason is that i play with straken so my command squad is either moving towards the enemy or in cc so there is no time for the MOO to shoot.
I like the astropath very much because in my eyes the ability only costs 15 points (15points you pay for a bodyguard as well who also is 1 wound) and that is a bargain for the ability to get your reserves on 3+ turn2 and reroll flankings.
fleet officer is not to my liking because he works against my playstyle. I rather have all my oponent on the table turn 2 than worrie about that dropping multimelta-speeder for another turn! on the other hand allmost noone in my area uses outflankers so I dont use that part of him either.
I typically run 2 command platoons, 1 moves forward with the troops issuing orders, the other stays back with the MoO, issueing orders to hw teams. I find using two advisors at once far too expensive, and as far as the MoO goes, I just can't say no to a 30pt large blast template!
The other two simply do not suit my play style.
I've rather my reserves show up when the enemy is mid field, and I want as much of my opponent on the field as easy as possible (execpt in planet strike games!)
Oh god no, not at all. The squad can't move if you want him to fire, and he can't target independently so the squad has to forgo shooting if you target anything more than 24" away, which you will want to because it scatters like mad.
OoF on the other hand, forces your enemy to come at you piecemeal, which is suicide against Guard.
I guess my opponents no longer put much in reserve for it to matter to me