As in every season, people are probably soon going to ask "what's the easiest conquest?" since everyone needs two of them for the stash tab in the Season Journey. Because I spent quite a lot of time preparing (and finishing) this stuff, it would be a shame to let all this info go to waste.
TL;DR: The easiest bounties are Boss Mode and Curses!, followed by Years of War and Master of the Universe, while Divinity is the hardest.
I've written a guide for the Boss Mode conquest a few seasons ago, check it our here. It's definitely one of the two easiest conquests, the only question is if it's easier than Curses! - and the answer probably is "it depends". Are you in a group with 3 or 4 people? Then Boss Mode is easier. Are you a Demon Hunter or Monk? Then Curses! is easier. For 1-2 player groups and the other classes it depends a lot on your power level, whether you see your strength in single target damage and movement speed or burst AoE damage. Give both a try. Boss Mode is one of my favorite conquests because it involves a lot of planning, coordination, and execution for doing it early. And it can be a lot of fun, it's like a mini speed run. Also: It ticks off quite a few boxes of the Season Journey!
The guide for Curses! is as follows: Open a game. Go to Act V, Path of the Drowned. Check for the "Cursed Peat" hill. If you find any other event left or right to the waypoint, leave. If the bounty is another event (like King of the Hill which looks like Cursed Peat, but it's not), leave. For the Cursed Chest you might want to drag up an elite and kill it before the start to proc Bane of the Powerful. Other than that it's burst/AoE test, and as such the Uliana's monk and UE DH really shine here, whereas other classes might have to push a bit more (or even get help).
Clearing a GR75 is, in my opinion, the most boring conquest and I don't know why it comes back every season. This one does not require planning, it has no coordination aspect since it's 100% solo, and it's just a pure power check. Hope for good drops, get paragon and gem levels, and then fish for a good rift. Done. For guides check the Builds section and just follow the cookie cutter spec of your class. Bleh.
Nevertheless, many people do this because (I guess) copying a cookie cutter spec is easier than put in some thinking into the other, much more fun conquests. In terms of power level one thing has to be said: by the time you finish Dynasty, you absolutely have the power to finish every other conquest that has ever existed (in this season or any other). Of course you need to get the 6-8 sets for the conquests below, but if you gamble for those early on you'll complete those sets before your character is powerful enough to clear a GR75.
Years of War and Masters of the Universe
Now, I know many people are hesitant to do this. "Farm 6 sets? CBA." - "Set dungeons? I hate those." - "I don't like a second class, go away." What bothers me about this is that everyone is crying for diversity, and this conquest is offering it. You get to play 8 different sets, 8 totally different playstyles, enjoy the insane diversity and brilliant balance that this game has (yes, some sets are 3 GR tiers more powerful than the others, but balancing 24 sets the way Blizzard did in D3 is unprecedented in any APRG). Therefore my advice is: Do those conquests! You might find to enjoy another build you haven't even thought about yet.
When planning this, I wanted to stick to one main stat, so the choice was between Wizard+WD, Crusader+Barb, or Monk+DH. Since two of the fastest farming specs are the Sunwuko Monk and the UE DH, I went for the dex classes. I played a monk, my friend DH. With lootshare it took us about a day to get all sets completed. Keep in mind two things:
1) If you plan ahead, you can one-shot the set dungeons, which means you can "share" one set - as long as it's not enchanted, you can share it for two hours. So we put all set items that we didn't need for our farming spec (Raiment, Inna, Shadow, Marauder) and some useful non-set items (immunity amulets, class-specific items like Sweeping Wind belt, quivers, and so on) on a twink as soon as we found it.
2) Keep *every* set item, even if you have it already. Remember that you can re-roll pieces! For example, I desperately wanted a Madstone for my Uliana farming build, but also get the Sunwuko set for farming. I got no less than 6 Sunwuko heads while gambling for the Madstone, and re-rolling those gets you the entire set. It's also the only reasonable option to get jewelry/weapon pieces of a set (Sunwuko, Inna, Natalya).
Overall, Years of War (6x GR55) is much easier. However, you can "cheese" Masters of the Universe if you get help. (If you don't I would certainly recommend adding a 3rd class to the list, like Barbarian, because you want to avoid Uliana's solo set dungeon at all cost). In the following Google Spreadsheet you can see the spec we used, some item suggestions, and do's and don'ts about both conquests:
If you go for it now you might still get high spots - on NA some of those leaderboards have only 2-3 entries, so top 10 is possible! On EU you can easily get top 100 on any of those as there are about ~10 who have done them. We got the worst terrible loot imaginable and a really bad start into the season with team members bailing out a few hours before the start, and I still got rank 4+15.
Last but not least, thanks to Eramir for tagging along for the fifth time, keeping the spirit up despite me ragequitting (lesson learned - don't do the M6 dungeon at 5am) and to AliRawidDuh for all the discussions about preparing and planning (though you can't plan RNG apparently... talking about "completing UE set 3 hours after hitting 70" vs. "600 paragon levels without an In-Geom or Madstone" ;-))