Lower brackets are a bit tricky in terms of PvP, because there are a lot of nuance talents that if you're up against really skilled players will make a huge difference, but against normal noobs in BGs, they won't do anything.
Personally at 45 I'd suggest something like the following, by main spec:
BM This spec is kinda interesting to me, mainly because you're just high enough to take both focus generating talents, making your pet into a complete beast in PvP. A true caster killer if there ever was one.
The build is for level 46, but if you want to spec it at 45, leave off a point in GftT.
MM This is a pretty great build for PvP, as it's close to maxing your Multishot, probably the best PvP shot in the game. 5% HP from Endurance Training is essential. Scatter Shot provides a great out against Rogues stunlocking you and a nice interrupt against casters that get in your dead zone.
SV This is the start of the build I currently use (check the armory link in my sig for my spec), and I find it really great for PvP. Lots of HP, Deterrence for Rogues and Warriors, and Entrapment and Imp Wing Clip are very strong talents.
I'd try these three, see which you like best, and go from there with one. These are all very strong in PvP, and the SV one will only get more powerful as you level (you'll have 5-7% more crit than most MM/BM Hunters at 70 with SV, and the most HP you can possibly get), once you start getting talents like Scatter Shot and Killer Instinct/Lightning Reflexes.