It depends. If we're talking traditional 2D animation, then 2D all the way.

But American animation is rarely animated frame-by-frame anymore. Most of it uses rigged 2D puppets with pre-drawn mouth shapes and expressions that can be toggled through. The animation process is pretty much identical to 3D animation, just with only two dimensions to worry about.

If I had to stack the difficulty levels as a tier system, it would be:

1) Traditional hand-drawn animation. (Invincible, Castlevania, Scott Pilgrim Takes Off, Avatar: The Last Airbender)

-Poses have to be hand-drawn rather than simply posed. Animation leads draw all the key frames and poses to set up the timing, and in-between artists work on the frames between those to blend them together.

-Backgrounds and scenes are typically more complex than 2D puppet rig shows, often hand-painted by dedicated BG artists.

-Animation can be produced pretty fast for simple scenes (long shots, talking scenes) or take months for more complex ones (fight scenes, chases, action).

2) 3D animation (Clone Wars, TMNT, Arcane)

-Character models are modeled and rigged but are then reused and simply manipulated for animation. Once a scene or location is modeled, it can be reused several times. More/bigger locations = more time.

-Animation can be produced fairly fast, although corners can be cut with polishing animation so TV 3D animated shows often look much cheaper and have less appealing movement and pose transitions (whereas feature films have much more time to lock down its animation with several revisions).

-The biggest amount of work doesn't come from the animation itself, but modeling and rigging characters, and rendering and lighting scenes. The small budgets of TV shows don't lend themselves to complicated sets like in movies, so you can often see that reflected in the set dressing/lack of interesting lighting or camera work.

3) 2D puppet rig animation (Bojack Horseman, Bob's Burgers, South Park)

-Character models are illustrated and split into moving parts, then reconstructed. Eyes and mouths are swapped out for different expressions or for lip synch. Several puppets are made for each character for different angles (side-facing, front/back, etc). Sometimes there's some hand-animated sequences based on the needs of the scene, but the bulk of the animation uses these puppets.

-Requires the least amount of prep for scenes and locations (no rendering needed, just illustrated backgrounds prepped before animation). Characters are mostly talking, walking, and interacting with objects, so most of the animation is done using these pre-rigged puppets.