This is this first part of an overview of Sisters of Battle wargear, and so focuses on the guns since that's the most important part of a Sisters of Battle army. One thing to point out is that these weapons are scored subjectively based on being used by Sisters of Battle, rather than other armies that might be able to take them. So while a Multimelta is damned good for Space Marines, it's only an average choice for Sisters, who don't have the luxury of standing still within 24" of the enemy.
Bolter 2/5: It's a bolter. Bolt shit.
Bolt Pistol 1/5: Only Seraphim ever need bolt pistols. The only advantage they have over bolters is the extra attack in close combat, and the only units you have that should ever be in close combat can take inferno pistols.
Storm Bolter 2(3)/5: Flamers are better, meltas are much better. Why take it? This is your psycannon bolt platform. Given to an Inquisitor, this gun kills Eldar of either type dead, and does a good job against most other things as well. It also gives your Blessed Weapon/Eviscerator Heroine something to shoot with their BS5 too.
Heavy Bolter 3/5: Good for retributor squads. which is good since they're the only infantry that can take it. Sisters have problems with infantry at range, and this is their only answer. Putting it on an Immolator, which is the only other platform for it, is a waste.
Melta Gun 5/5: Your bread and butter anti-tank. Take them. Take them lots. They are especially good in dominion squads. Almost all special weapons in your army should be melta guns.
Multi-melta 2/5: How can a bigger, better melta be worse? Because it's heavy. Not good on Immolators, where it's a 20 point upgrade, unless you feel like fielding six Multi-melta Immos (don't). Sisters rely on mobility more than most other armies, so should never be given heavy weapons unless they have a lot of range.
Inferno Pistol 3/5: This gun depends on who is using it. NEVER GIVE IT TO SOMEONE WITHOUT A JUMP PACK. 6" range with a 3" Melta is unforgiving. Seraphim make good use of them, since they're twin linked in Seraphim squads, Not bad on heroines, but if you're taking an eviscerator it's a bit moot, and you don't get the extra pistol attack.
Flamer 3/5: OMG Flamer. Sisters' propensity for for mobile war loves the flamer. On dominion squads in an immolator it turns infantry of almost any type to mush. It's less effective against MEQ, but still okay.
Heavy Flamer 5/5 (6/5 on Immo!): Everything a flamer does, but better. Wounds most infantry on 2s or 3s, ignores cover and most armor (stupid beakies), and it's an assault weapon for your heavy weapons slot, essential for that mobile fire sisters need. On an Immolator it becomes the very deadliest gun in the army since it rerolls wounds and can be given Blessed Promethium.
Plasma Pistol 3/5: Not bad on a Seraphim Superior, a decent choice for your Heroine or Inquisitor. I usually ignore it in favor of something that's not gonna kill me.
Combi Weapons 2/5: Good for that one extra shot of Flame or Melta, but since only sister superiors can take them, not really worth it. The Combi-Stake Crossbow, however, is worth note for its ability to tear Hive Tyrants to pieces.
Hellgun 1/5: Only for Inquisitorial Stormtroopers and Warrior Retinue, and that's about the same as saying not good at all.
Laspistol 0/5: If your priest doesn't have an eviscerator he'll have this, as will most of your Inquisitor's retinue. Damn shame, really.