they're all about the same. let's not pretend any of them are better than any others.
No way.
Diretide - The game mode before the grand Rosh fight was kind of meh, but the actual Rosh fight with people figuring out ways to do it faster/more times was pretty cool. I like that idea and was glad to see it come back in the Year Beast fight.
Greeviling - I understood the idea behind this one, but it felt kind of pay to win, as the Seraphics had a pretty clear advantage here. The idea of bringing your eggs from Diretide into this thing was neat, but then it was pretty unbalanced in favor of the people that had more stuff for the better greevils. I was stuck with Pudge Rot, Enchantress Nature's Attendants, while my opponents have....Pudge Hook and Black Hole?! Seriously? The items were given out completely randomly, and it just felt kind of all over the place. There were some strategies to steal the big greevils (Sniper Ult for instance), but it still mostly relied on who had the better greevils. Not a very fun game mode and I was sick of it after about 2 games of it.
Wraith Knight - A straight up defense mode that was simple, fun, and had cool variations on the heroes. Storm Vortex pulled multiple enemies, Windrunner Shackle hit multiple targets, and much more. The different difficulty levels got progressively harder, and since you always saw the same waves in the same order, people liked to prepare strategies for the New Game+ mode and higher than that. Also I definitely got a chuckle at the time when it said "YOU DEFEATED" after beating it. They had the new items, some were upgraded versions of items we already knew, and some were something completely new. Our first taste of something made by the custom game tools.
Year Beast (boss fight) - Valve clearly saw what people liked about Diretide, and why they wanted them to give it back (other than hats of course): Preparing a strategy to take down a boss. People would pass around team comps they would use in places like here or on Reddit. There were bugs with it, and changes every day or so as new exploits or super OP combinations were found. I specifically remember the Visage exploit where you could fly a Familiar in the corner of the map and it would just try to chase it forever, allowing you to hit it as long as you wanted with no risk of taking any damage. It did have a bit of pay to win with it, since you had the ingots to use as gold in it. I remember another strategy that was used after the Visage thing was to just buy those fireworks that stunned it, and just spam them until you hit the damage number you wanted. The moves were interesting, often telegraphing what was coming next, and even stuff like basically a bullet hell shooter.
Year Beast Brawl - They went back to a variation on regular Dota, but this one was definitely more pay to win, but not quite as much as Greeviling. Some of the stuff you got on the higher upgrades was ridiculously OP, like the Refresher Aura with Zeus. It was okay but I got tired of being on a team with people that didn't have enough points. CM Arcana buyers had plenty, but everyone else not so much.
I really hope that if Valve ever does another event game mode, that it not be so reliant on paying to win. Wraith Night was the best simply because there was absolutely zero pay to win elements to it.