The final entry for the elites section is the rapier weapons battery. It is intended to be a semi-automated portable weapons platform. The core concept is that it is able to be deployed where regular tank platforms would not. With a decent toughness and two wounds, the rapier carrier should hang around for a while on the battlefield and is therefore a viable option for the types of terrain its supposed to transverse.
There are only 4 options for the weapons, but each force organisation chart slot may take up to three of these platforms each. To be honest, I think we'd be wanting to take 3 of the same type if we decided to field these batteries: same range and same purpose makes for a better squadron than a mixed weapon squadron.
The weapons are: quad heavy bolter, laser destroyer array, quad mortar (a.k.a. the Thud Gun!), or a graviton cannon.
The quad heavy bolter is only heavy 4. Although cheap, it is fundamentally still a heavy bolter kind of weapon (i.e. AP5 and nothing particularly special about its effects). This is not my preferred load out as I think infantry can do this almost as well for similar points.
The laser destroyer array is a great tank killer. Taking three of these would cost 165 points and give an excellent possibility of taking out some of the heaviest enemy tanks on the battlefield. Albeit with a 36" range. After that, targeting heavy infantry or primarchs seems like a good secondary idea! The twin-linked and ordnance can really help with this role.
The thud gun is a bit of a classic. With 4 shots and a reasonable chance of pinning (thanks to the "shell shock" special rule), this can really do the hard work against enemy infantry blobs. And the excellent range (up to 60") is nothing to be sneezed at either. I almost prefer this one for sentimentality stakes, but I'd probably still take the laser destroyer array to be honest.
The graviton cannon depends entirely on how you feel about graviton weapons. Rolling a toughness test is a great mechanic that can help bypass regular Strength vs. Toughness rolls. And haywire vs. tanks gives excellent tactical flexibility.
So overall, I really like the laser destroyer, but could certainly opt for the thud gun as well. The other two depends on how you feel about graviton guns or heavy bolters and can therefore be much more situational. I could easily see the graviton cannon getting plenty of use, but less so for the quad heavy bolter.