non-military type here, but from the posts I've seen it looks to me like you don't mind...
For canon weapons, any battlecruiser will do. It might not be intended as a defensive weapon, but would work better than anything else I can think of.
For non-canon weapons, any Battleplate would be perfect. ( ovalkwiki.com/Battleplate ) They're actually designed for defense, even if only officially.
More seriously, I think the common sense answer is whichever quick, handy weapon(s) would best suit the situation. It would almost certainly be something handy and quick to bring to bear, like any of the PDW-intended weapons lots of people suggested; unsurprising, since PDW is exactly the role we're talking about. Beyond that, it would depend on the environment and expected threats. I'm guessing that in a BT setting the only thing we'd realistically have to plan on defending against _with a weapon_ would be other humans equipped with less than power armor; anything more, and there's probably no point bothering... I am almost totally unfamiliar with BT small arms and how they work, but I read about Needlers and am aware that lasers coexist with guns more or less like they do for heavy weapons. If laser small arms don't have any particular handicaps, I am thinking whatever kind of laser is most analogous to an SMG... and/or perhaps shotgun and/or IAR depending on how they work. Ideally, it should be suitable for whatever levels of body armor might be likely. Since a lot of people would count a knife/hatchet/machete/E-tool or such as a weapon, I guess I ought mention it since it's likely enough to be essential that if I had to choose one "weapon" I might go for the shovel. I imagine it with a radio and flashlight built-in, and the flashlight might be very handy for personal defense anyway.
That reminds me of something kinda tangential I might like to ask of "military folk." With how the D-Cell-size flashlight is such a valuable defensive tool in the civilian sphere, what do you think a more purpose-designed "defense flashlight" should be like? I'm thinking more like a modern diode-based torch (maybe red night time light on on/off switch and bright-*** high-colour-temperature on momentary button?) built into a handguard that doubles as a fist-loading weapon, so it's smaller, handier, easier to retain, and better protects the hand. Am I mad, or am I onto something? Ironically, it would probably end up being illegal in a lot of places where a more unavoidably lethal option would be legal...