I guess it depends on what level of play you are talking about. In terms of learning the basics and becoming somewhat proficient in a style, I think 2A/3A are definitely the hardest.
3A has some complications that can make it more complex/difficult to learn. For example, it can be difficult to throw/keep the two yoyos parallel, especially when throwing breakaways. It is easy to get the two strings tangled up, which can lead to more time spent untangling them than actually practicing. When both yoyos are mounted on the strings, movements in one hand or with one yoyo can jar the other yoyo around, so you have to learn to control that. On the other hand, 2A has to be learned from scratch. A lot of the skills from the other styles transfer well between styles (i.e. learning 1A will help a lot with 3A), but that doesn’t really work with 2A. Because of that, 2A can be the hardest style to learn for a lot of people.
Freehand vs offstring I think depends on personal skills. Offstring doesn’t have the level of multitasking that freehand does, but it has its own difficulties. Offstring is unique in that any missed string at all can completely kill the trick. Maneuvering the yoyo can also be more difficult with larger yoyos, which is why hardly anyone uses offstring size yoyos for anything else. The extra size helps with making offstring catches more consistent, but it hinders most other maneuvers (because of the tradeoff, some offstring players use smaller non-offstring yoyos–Kazuaki Sugimura used to use FHZs, for example). Offstring is also unique in how the yoyo interacts with the string, so you have to completely rethink how you maneuver the yoyo. I think most moves and concepts from 1A adapt more readily to freehand than offstring because of that.
So I think in terms of what is simplest to learn, the tiers would generally go:
with the styles on the same tier going either way depending on the person.
In terms of the top levels of play, I don’t think it works the same way. 1A is simplest to learn, but the top 1A players take advantage of that by pushing their tricks further. In some ways, I think it is kind of like comparing piano to other instruments. Because the piano is relatively simple to play, masters can play incredibly complex pieces of music that might be impossible on some other instruments. Likewise, 1A players create complexity within the style by doing things they could not do with the additional restraints of the other styles.
Because of that, I think a great 1A player has as much skill as a great 3A player. 1A has the deepest field of talent and is usually the marquee event of competitions, so you could probably say it takes the most skill to reach the apex of 1A play right now.
Another way you could look at the issue is how restricting are the restraints of a style on the tricks and performances of the top players. In terms of speed, style, flow, presentation, complexity, etc, I think 3A has always lagged a bit behind the other styles. That’s probably an indication that 3A is inherently more difficult/complex, because that difficulty has a greater effect on the performance elements and trick composition of top 3A players than other styles.
Of course, this sort of thing is also affected by how many players are devoting themselves to a style. 2A, for example, has always been at the forefront in terms of presentation elements even though it is very difficult/complex because it has a much longer history of advanced play than the other styles. It used to be that 2A was the top competition style and 1A was a secondary competition. By the time other styles were emerging, 2A had already been pushed pretty far by top players, so it’s hard to compare on that basis. Likewise, I think freehand has been pushed relatively quickly over the past several years as more people have gone into the style.