Indeed the Emperor is a good perennial choice for a BB. Alas, as the official rules stand it is the only good IN BB. This is why I recommend house rules for the Apoc and the Oberon. I also recommend house rules for the Retribution too, but didn't mention it or the Emperor because you didn't bring either up. I have the outlook that to play BFG using completely official rules is excruciating, so I'm all for house rules and don't see why someone shouldn't be able to take fluffy choices like the Apocalypse and Oberon in an Armageddon fleet.
It should be noted that the Oberon originally had 60cm guns at its present cost of 335 pts. At the time the Emperor was only 345 pts (and the Retribution was 365 pts). With continual complaints that the Retribution was rubbish (still is) and the Emperor was fantastic (still is) the HA elected to swap their points values as a final solution, making the Emperor 20 pts more expensive and the Retribution 20 pts cheaper. Before these changes the Oberon was balanced compared to the Emperor, not as good but cheaper and definitely playable. Instead of simply adding 20 pts to the cost of the Oberon just like the Emperor the HA decided to drop back its range to 45cm instead. This came completely from left field. No one complained about the Oberon, there was nothing wrong with the Oberon and it functioned in the same manner as the Emperor. The HA broke it.
At the same time the HA increased the cost of the Armageddon from 235 pts to 245 pts, again, a complete surprise to the community, as it was just feasible at 235 pts. It took considerable campaigning to and argument to get it reduced back to a tolerable points cost.
Also, the original Blast Marker rules stipulated that only fire that traced a line through the actual BM was subjected to a right column shift. After the massed turret rules came out there was an introduction of a shared shield rule (designed to balance the massed turret rules) which allowed for the movement of BMs off their direct line. This made it possible for beardy players to take advantage of the fact and move BMs out of the way of subsequent fire, therefore gaining more efficent firepower. In response to this the HA ruled that a BM in base contact counts as being in contact all around the base, therefore all incoming fire would be effected, regardless of whether it passed through an actual BM. This change reduced the effectiveness of WBs (which were balanced just fine) as well as reduced strategic play (using a flanking force to drop shields away from subsequent lines of fire used to be viable). This change was retarded.
The upshot of all this is that if I were you I'd use house rules. Reading these forums will give you an idea of where imbalances lie and what fixes should be used. Fan files such as BFG: Revised are also a good guide (though I will admit that these seem to go beyond their original remit).