I mean the simple truth is that there is no "SP BiS". As with most spec's your gear drastically changes due to situation or gameplay. I find this conundrum is even worse with shadow priests due to the fact that many builds are widely argued. Im assuming your looking for a sort of "Go-to" BG orientated hybrid SP gear build. But even that is debated upon, the question of the values of crit in bg-play, or whether you want a little more surv (the longer your alive in shadowform is more dps/utility you put out) are all decisions you will have to make yourself.
Obviously some gear will never be replaced as that gear in any situation is "BiS" (Hotshots, etc.). But you will quickly find that a majority of gear, and most of your chant choices are throw-up choices completely depending on situation and further on personal preference.
An example of this is your belt slot. While many, including myself, agree that
Nesingwary's Sash - Item - World of Warcraft is in almost all cases BiS, many others run
Girdle of Nobility - Item - World of Warcraft because of the extra crit.
Drayner's ally priest is a perfect example of a "True" glass cannon build. That would mean a full int stack, while my priest (missing Hotshots sigh) runs a few bonus stam pieces because I believe that an overall surv boost beats glass cannon dmg in BG's anyday (opinion).
The first thing you should be asking is "what am I supposed to be doing here" then build your gear around that idea. Tinker between int stam and crit until you yourself find a nice hybrid build that works for your play style.
Yes, some gear is BiS no matter how you look at it.
This gear includes Hotshot's, Slippers, Overlords Neck, Spidertank Oilrag, BoA legs, shoulders, cloak, and weapon.
Your CHOICE pieces include, offhand, head, belt, rings, trinkets, chest (Mechbuilders are honestly a waste of money this shouldn't be choice don't do it the int is comparatively worthless)
Trinket choices obviously depends on your race (humans get free trink, undead basically get free trink).
Savage (Siccness) has in my opinion an awesome build going on, and Drayner will be your go-to TGcannon build. I wouldn't recommend running my current armory build as its completely based off the arena war-games I have been running and is fully personalized.
Sorry for the TL;DR but they are complexities worth mentioning for any person searching for "BiS" builds.
Happy hunting,
Pattern