Out of all small PD options, Vulcan is the best (for personal PD). A single one can stop missiles. Maybe not reliably, but better than almost certain failure from a lone beam against anything bigger than a Swarmer SRM. Beams... most have low enough DPS that missiles ignore the beam until it hits the ship. (Burst PD with charges is an exception, but costs about twice as much as Vulcan.) I consider pirate Buffalo the best Buffalo because it can mount a Vulcan instead of a lousy beam.
Light Autocannons have the advantage of no windup, not too high OP cost, and maybe better availability. Sometimes, the better light kinetics are not always useful, or that intended ship is likely to die and replacing the weapons is harder than the ship. I would not use railgun or needler on Lasher because Lasher is likely to die, and replacing the weapons is too hard. I would use autocannons on a Lasher instead because they are more expendable.
The main advantage of Arbalest over Railgun is availability. Arbalest is common as dirt, and available even in open market. Railgun is rare enough that I barely find enough for all of my ships. Both are comparable, but railgun is the better weapon overall, admittedly.
Light needler's main advantage is range. No other non-beam light weapon has as much range as light needler. Light Needler is one of the rarest weapons in the game. I never find enough for all of my ships, and what I do find gets saved for special playerships where there is no substitute for light needler. Also, if the player has Target Analysis 10, then kinetics do enough damage to armor that an all kinetic configuration is viable (not necessarily optimal) for all-purpose assault.
Dual flak is not only "the best point defence in the game for its price.", it is the best PD in the game! Nothing else competes. It puts even Guardian PD to shame.
The main reason to use Heavy Needler is the player cannot stand Heavy AC's atrocious accuracy and turn rate, and has lots of OP to burn. Needler used to be great when it had much more ammo than anything else, but since ballistics no longer use ammo, Heavy Needler is overpriced for what it does. My go-to medium kinetics are either Heavy AC or HVD.
Never been fond of Mark IX. It's accuracy is bad, and the DPS is weak. However, it is effective enough if I have nothing better, and (I think) it is available at open market.
Thanks to Storm Needler's perfect accuracy and high DPS, it is effective against armor. The main problem is flux cost. When it used to be 500-something, it was all-powerful like Mjolnir is now. Storm Needler cuts through shields, drilled through armor (if you can focus on a single point), and killed! It still had its other weaknesses, but something like Dominator or Onslaught could mount Storm Needlers and kill things fast. Now, with high flux cost, it is not possible to fire them for long. The only ship that could use multiple needlers effectively, Conquest, is too fragile to trade shots other capitals. Storm Needler works best when it can fire non-stop, but the high flux cost does not make that possible. Also, 800 range is lousy for a heavy ballistic. It was okay back in the day when it could be fired long enough to kill everyone, but not today with high flux cost.