What is THE most effective bicep curl for gaining mass?

In my opinion it's probably a very simple formula:

The Bodyweight Barbell Weight

Step #1. Sit flat on your back, with your elbow bent.

(or press your butt into your bench)

If you have a good set of dumbbells or squat plates you can place any size object into the bottom of the bench, it will absorb the weight. If you don't, then you must sit upright as with any upright exercise.

Step #2. Place an object straight in front of your chest, at least 5 or so from your chest so they touch. If they are close, then you can still press them, but you can't press them harder or with more force. (I've taken pictures of some of my most popular curls and do not hold back.)

Step #3. When your object passes directly in front of you you should use your back to support the weight, and pull your upper arm down until your arm is at a complete 90-degree angle to the floor.

Step #4. If you feel a bit of a stretch at the very end, slowly roll your lower arm forward in your chest, bringing your arm above the center of your chest. Be able to see your weight on your upper arm and hand back.

bent knees prevents you from using your hips as a hinge

I didn't see any decent gains in my biceps until I did isolation. Weighted pull-ups really hit my lats more than anything

Thanks. I assumed it put some extra strain in your core or something. Assuming one's intention is not to cheat like you said, are straight legs ''allowed'' or is the form generally looked down upon/ not accepted as valid?

The one you're not doing.

In appearance or in actuality?

Bicep is two muscles (hence bi).

Strict form curls tend to grows the width of the bicep.
Incline curl are harder version of normal bicep curl makes it harder/more effective exercise by shifting the gravitational directional point (get more use out of bicep).

Hammer curls tend to grow the "peak" more.

Actually no

BB = biceps brachii
BR = brachioradialis
EZ = ez bar curl
DC = dumbbell curl
>"We detected higher activation profile of both BB (P < 0.05) and BR (P < 0.01) during the EZ compared to the DC. "

peerj.com/articles/5165/

This is true.

However, TIME is a finite resource.
And there is opportunity cost.

I prefer to maximize and combine as much as possible to optimize time spent.
This is why I tend to do all exercises standing (gets core involved) and curl with both L+R at same time.

Think about it...40lb curl x2. That's 80lbs that my body is counteracting and stabilizing while standing. Compared to preacher curls (sitting).

Incline curls are super effective though and I'll generally do them as dropset.

Were your palms facing you?

coffee is good for you

I did both palms out and palms in.

Bicep curls with a bar are easier to overload the bicep directly. When doing pull-ups my lats take over.

Do your curls with your palms always facing upward to prevent your forearms from doing half the work, our lord and savior Jeff Cavaliere has a video about it

Sandell curls :^)

This isn't an opinion, we can clearly see a scientifically correct answer:

First we consider the following axioms:
1. The visual "bicep" size is comprised of the brachialis and the biceps brachii
2. To maximize size, we want to maximize training efficacy of those two muscles
3. The bicep is the primary supinator of the forearm
3a. The biceps brachii is in fullest contraction with the hand in a fully supine position
3b. The brachialis takes over when the hand is in a prone position

Thus we can see it's obvious that the best method for training bicep size is one which trains the two heads maximally. In order to keep full tension on the muscles as they travel through the lever motion of a curl, simple gravity is suboptimal; we need cable tension. This means that the optimal exercises are
A) Cable curls (or curl machine) with flat bar
B) Reverse cable curls (or curl machine)

However, in a reverse curl, the brachioradialis is often the limiting factor of the lift, rather than the brachialis. Thus we should make a compromise here, and B becomes C) Hammer grip cable curls (or curl machine)