So, what's the top picks (siphon, bullwark, concussive)?

In my opinion and how I play in the game, I would say:

Siphon Weapon: Always take one no matter the difficulty. Even as your sidearm (secondary weapon).
The best ones are:
1. High fire rate with low damage, but high blind fire accuracy (It's no good if you're firing into the air most of the time, as it needs to deal actual damage before it can restore your health). Charger, Hurricane or Raptor for example.
2. Low fire rate, but with acceptable damage and AoE. (This is so you can fire without needing to constantly re-aim all the time and target small groups for health restoration, very important if you're using a controller like I am). Scorpion for example.

Concusive Weapon: Only take on Gold difficulty or less. Only as your primary weapon, and only if you're REALLY good at Headshot/Weak Point targeting.
The best ones are:
1. Sniper Rifles. This is because you can actually aim for the head/weak point most of the time, unlike other weapons which will unlikely happen most of the time. Black Widow, Widow, Isharay, Shadow for example.
2. AoE weapons. This is because it looks like the AoE can trigger the Biotic Explostion as well assuming it's the kill shot (likely the AoE is considered for the head/weak point hit for triggering). Scorpion for example.

Bulwark Weapon: This should be the default weapon if you have it. It'll always give you the DR assuming in range of others. Otherwise, no different from a normal weapon. So the normal weapon is not as useful in that sense. Especially important in Platinum.

In short, "any" weapon as your primary with a Siphon secondary should be the goal for everyone I feel for Gold or lower difficulties. Bulwark and Siphon if you happen to be in Platinum.

That's my thoughts on the weapon variants.