Which is a favorite form factor depends a lot on the situation!
If you want 80 lumens for 12 hours with a NiMH battery config., the 3xC works well. A 2xD formfactor can actually also work.
If you want 500-600 lumens with 4-5 hour runtime, the 3x18650 works great. If you can live with half+ the runtime, you could do it with 2x18650.
If you want 1,100 lumens and can live with a 1.25 hour runtime, the 2x18500 formfactor is nice and versatile. You can also use it with a bigger reflector in a 3xC123A to get big throw.
If you are inside working with normal office type lighting, and you want a very small EDC type light, the 1xAAA nowadays is a great light.
If you are traveling and weight and size and battery availability and versatility are important, you go with the 1xAA and 1x17500 formfactor.
If you are outdoors and want a medium sized flashlight, the 2xC123A or 1x18650 (or both) formfactor is quite versatile, especially with a 2 mode setup (max blast plus very efficient useable light).
If you need to be hundreds of feet below water and want a locator light (Glo-Toob), it's a 1xC123A, cylindrical shape.
So what is the favorite? If you want to weigh by use time, you probably use the indoors back-up EDC 1xAAA the most, followed maybe by the outdoors 2xC123A or 1x18650, followed by the travel 1xAA or 1x17500, followed by the big throw 3xC123A or big lumen 2x18500, followed by the long runtime (power outage!) excellent lumen 2x18650 and 3x18650 or even 3xC formfactors.
The old stand-by's like the 3xD (or even more than 3 xD) Maglite lights are obsolete. Maybe the 2xAA is also quite versatile for indoors/outdoors uses and could substitute for the 2xC123A or 1x18650 formfactors.
Then various specialty lights at pretty small rates of useage.
So, all in all, "favorite" formfactor is not a very revealing category, until you link it to use and time of use...
Oh, one more thing, how do these fit into your hand?
Basically, without disability, the human hand can easily work with the 1xAAA, the 3x18650, and everything in between (we worked with 3xD, 4xD, etc., before, didn't we?). In terms of natural fit, probably the 2xAA and 2xC123A are the most natural.
One formfactor that, strangely, does not appear in the above list I just realized, is the 1xC123A (putting the specialized Glo-Toob aside)! I guess it tries to do everything so really it gets beat in every category by other lights. You don't really need it indoors, since a 1xAAA can do the job with much less bulk and weight. Outdoors it is not quite enough, the 1.25" reflector is a kind of minimum. Also the power and versatility of the 2xC123A or 1x18650 also outcompete the 1xC123A outdoors. On travel, the 1xAA and 1x17500 is superior and more versatile.
In a way the 1xC123A is a dilettante's favorite light because it seems to show sophistication and one-light-that-can-do-it-all-ism, but deep down, it does not!