The Apocalypse class makes Necrons and Space Marines strongly consider purchasing 365 pairs of underwear so as not to reuse ones that they have filled with terror-feces. Also, there's an actual model and it looks fantastic. The Apocalypse is also used in two great lists. The Victory is only used in one (admittedly fantastic) list, but it is a bit more reliable and has the torpedo option, and more options means more flexibility. However, it does have 2 fewer lances on each side, which reduces the intimidation factor.
To be honest, they're both good. Choose one of the fleet lists that tickles your fancy. The Segmentum Solar lets you mix in Space Marines without having to resort to Allies. The Segmentum Obscurus Bastion list lets you take Grand Cruisers and even Chaos ships if you want! Finally, the Segmentum Tempestus list lets you have turrets galore, Lord Admiral Rath, and Admech ships.
Pick one of these lists, and then your choice of Battleship will be clear.
Do you own any other battleships? Choose a fleet that lets you take as many of your ships as possible without using Reserves. Taking a Battleship as a Reserve is only an option for the largest games (as you need to take 3 battleships to take one as a reserve).