This is a great question, thanks for asking!
So for me there are a few things that make an event great and a lot of it falls into what makes a great puzzle. I usually break down what makes a great puzzle into 4 categories: (1) Uniqueness, (2) Cleverness, (3) Tediousness, and (4) Depth. These categories are going to be in comparison to 3x3 as I think everyone would agree that 3x3 is one of the best events/puzzles.
(1) Uniqueness: for most good puzzles there needs to be a degree a uniqueness as, when lacking, it doesn't provide a reason to solve it outside of other similar puzzles. An obviously example would be something like the super sticker mods where you have to solve center orientation with solving the rest of the cube. This puzzle is nearly identical to a regular 3x3 and doesn't add anything to the puzzle/event. I don't really see any reason to solve this over a regular 3x3. Conversely, something like Square-1 is a puzzle with a high level of uniqueness. Its the only puzzle in the WCA that shapeshifts as well as having a unique, precise, and satisfying turning style. In addition to this, the design of the shapeshifting never hinders ergonomics so its rarely awkward to turn ( as opposed to something like the ghost cube ).
(2) Cleverness: cleverness is a bit more difficult to define but I'll do my best. I think the best way to explain it would to be to compare the uniqueness of the puzzle to the uniqueness of the solving experience. Again the super sticker mod would be an example of a non unique puzzle as well as a non clever puzzle. As stated above the approach to solving the cube doesn't vary at all compared to a regular 3x3. An example of a unique puzzle that lacks cleverness would be ghost cube. The shapeshifting aspect adds a lot of novelty to the puzzle, however, just like super sticker mod, the puzzle is solved exactly the same as a 3x3. A good example of a clever puzzle would be something like the latch cube or constrained cube. The puzzles don't differ from any way to 3x3, however, add a single new mechanism which completely changes how you solve the puzzle.. here being limited movement. They looks like a 3x3, they turn like a 3x3, but they are solved it a completely unique way. Although there are some shared ideas between 3x3 and latch/constrained cube they aren't particularly useful in solving these cubes. Because of the constraint you must solve the cube completely differently.
(3) Tediousness: something that makes for a poorly designed puzzle are those which difficulty arises due to tediousness (as opposed to cleverness ). Most tedious puzzle also lack uniqueness. Some clear examples would be ghost cube and higher order nxn events. Again ghost cubes difficulty does not come down to the cleverness of the puzzle but rather the obfuscation of the pieces. This ends up meaning that the puzzle just ends up being a pain to solve because the pieces are hard to identify. Solving the puzzle really comes down to a bunch of trial and error. In addition, once you are good a solving the cube it again just turns into 3x3. For higher order nxn events its a similar effect. Moving from 3x3 to 4x4 and from 4x4 to 5x5 each subsequent puzzle adds new concepts/mechanisms to the puzzle. For 4x4 you have to solve centers and pair edges for 5x5 you are introduced to things like freesilce which adds a new dimension to the puzzle. However, once we get to 8x8+, the challenge really just comes down to doing more of the same thing that you'd do on 5x5 or 6x6. There ends up being little variety in the puzzle and comes down to a test of endurance.
(4) Depth: this is a pretty straight forward metric.. how much depth there is to the puzzle? This is probably the most subjective metric. A classic example is going to be 3x3. There are tons of different ways to solve the cube in both forms of methods and in forms of advanced techniques. The WCA event with the greatest 'depth' would probably be FMC given the laundry list of approaches to optimize efficiency. An event that lacks depth would be something like magic/master magic. The solution is literally the same every time, zero thinking involved, just an execution test. Something like PLL times attack could be another example. 2x2, however, is a unique case that muddies the waters. Some people will characterize the event as 'just spamming algs' and 'requiring zero thinking' thus lacking any significant depth. However, for others ( myself included ), the depth of the event comes in the form of one looking the solve, considering multiple roots through the same scramble and choosing the best solution ( all in 15 seconds ). The characterization of the event is pretty contested so is mostly going to come down to someones experience with the event. As said above HD is a really cool method that challenges the former characterization of the event.
For me the best events will have high uniqueness, high cleverness, low tediousness, and high depth. Given that, heres the list:
3x3: A classic, the OG, doesn't need much of an explanation. Almost everyone loves this event, truly goated
2x2: See explanation under (4) Depth
4x4: The perfect balance of big cube stuff and 3x3 stuff.
Square-1: Most unique event with the most satisfying turning.
3BLD: One of the most impressive events. Amazing mix of challenge and speed.
Megaminx: 5x5 but with more 3x3 conceppts or just a better version of 5x5.
FTO: Perfectly meets all of the requirements above. Also super fun. #FTO for WCA
Oh yeah and the events should be fun ,duh. My favorites overall are 3x3 and Square-1.
anyway, I think that's it.
Cheers!
