This is going to be an analytics discussion, so some math is to be found.
Diablo 3 is a slot machine. You pull the lever (kill the baddie) and stuff pops out. Sometimes it’s good stuff but most times it’s crap. You need to pull that lever a whole bunch of times to get something decent. And the prize itself, while it might have a nice look, when you get to the meat of it, there are things wrong with it. Such are the RNG (random number generator) gods! For the purposes of this article, I’ll only focus on Legendary (or set) items.
In order to sway the odds, Diablo 3 has a whole bunch of number crunching below water. Enemy difficulty (regular, rare, unique, etc…) have different odds of dropping items. Game difficulty (normal, torment, etc…) have different odds of dropping items. Rifts and Greater Rifts have different odds of dropping items. Actually, Greater Rifts are still under debate, so I’ll scratch that for now – in particular because it takes 11:30 minutes to clear one. Some of these odds are additive, some are multiplicative. Let’s start with that concept first.
Additive odds are simple enough; you just put the numbers together. So 25% + 15% + 7% = 47%. Multiplicative odds are harder to get around. 25% * 15% * 7% = 30.7% and much less obvious. Sometimes you even add the odds and then multiply. You get crazy formulas.
Enemy difficulty is also part of this calculation, where non-regular (so Rare, Elite and Unique) seem to have a sliver of a chance to drop even a rare item whereas a pack will drop 3+ items. The quality of an item is dependent on if that enemy even drops something. So you can’t say a normal enemy doesn’t drop legendaries, it barely drops anything in the first place. You’re looking at a solid 100 enemies before seeing something.
Game difficulty and rift difficulty is as follows. Normal until Master gives a 0% boost outside rifts and 25% in rifts. Torment 1-6 gives 15% to 131% boost outside rifts and a 44% to 189% boost to drops inside rifts. So at any difficulty level, it’s clear that you should be in a rift, the increase is noticeable. Torment 1+ is required for level 70 legendaries. Torment 2+ has 50%+ odds of dropping a legendary in a bounty.
Game difficulty also increases enemy hit points, where Torment 1-6 go from 819% to 8590%. That’s a massive increase. They also hit harder but that’s less important for this point. You want to kill them fast.
Each build has a “clear on normal” build which is built on efficiency and speed. Or, 1-hit-kill specs. The clear builds mean you mow down everything on the screen quickly and get to focus on the elite packs instead. The bonus from this is the extra experience from the kills, increasing paragon levels which increases your power and makes you kill faster. So… yeah, killing regular enemies quickly is good. This brings up the concept of “clear speed” where you can complete an objective in X amount of time – typically this applies to Bounties (5 quests with a chance at a reward) and Rifts.
So that’s a pile of information. And you’re wondering, what the heck does this actually mean? Me too!
Depending on your leveling path, you either did the content in order or simply re-ran bounties a bunch of times. You hit 70, and finally started to use your Blood Shards with Kadala, gambling on gear. You focus on armor, getting some decent stuff. Unless you have a really crappy weapon, don’t bother gambling on one. Find a good clearing build and run a few normal bounties in Act 1 until you have a better weapon. Find a 1 hit kill build next and increase the difficulty from normal until you find one that is no longer 1 hit kill. There are great odds that this stays on normal and that’s ok. You’re going to reach a point where you’re 80% optimal on rare items. This means you have imperial gems slotted, you have 80% of the maximum roll on your primary stat (plus Critical Hit Damage, Critical Chance), some critical hit and damage as well as a decent (~8K) life on hit. Your weapon should have a gem slot and it should be an emerald. You’re now ready for Torment 1.
Actually, maybe you’re not. This is where the magic really happens is the Time to Clear (TTC, which is really similar to a TTK acronym). Greater Rifts (GR) have a 11:30 TTC, if you want to increment your rift level by 1 (for more loot). A TTC that’s lower means you need to leave the GR, so something else, then come back to kill the guardian. The “something else” should be Act 1 bounties, trying to get a Ring of Royal Grandeur – a legendary ring from the bounty cache. You should be able to clear all 5 quests in 2 GRs, depending on dungeon layout. Once you’ve completed the Act 1 bounty and closed the GR, leave the game and restart. GRs get harder over time, so your TTC will eventually reach and exceed the 11:30 and you’ll be unable to continue but have to restart. The benefit of GRs is legendary gems (should have them all in ~15 rifts) and a decent chance at a legendary on the boss kill. But you’re only killing the boss every 11:30 minutes plus. Due to this, greater rifts aren’t usually worth a whole lot until you can clear T6 rifts reliably – you’re not there for the gear but for the challenge and the gems.
The alternative, if you have a decent TTC, are regular rifts. Once you’re 80% optimal, you need to run Torment 1. TCC by that point is less gear dependent and more skill based. The question becomes, “do I move up in difficulty?” The answer is related to TTC. Let’s look just at the difference between T1 and T2.
T1 = 819%hp, 44% bonus to legendary
T2 = 1311%hp (60% increase), 65% bonus to legendary (47% increase)
So let’s say your TTC in T1 is 5 minutes. You get a 47% increase in rewards. The real metric here for choice is that your TCC cannot exceed 47%, or you’re actually falling behind. That means a new TCC of 7:23 or less. Other clear times are: T3 (10:13), T4 (13:31), T5 (17:09) and T6 (21:28). Now the difficulty climbs too, due to the HP increase. So your damage needs to climb as well. Let’s say you’re at 200K DPS. T1 to T2, to clear at the same rate, your damage needs to increase to 320K. From T1 to T6 you need to reach 2.1 million DPS. And that’s not even calculating the extra damage enemies will cause.
Given that you only ever get an average of X items in a rift, regardless of quality, you’re better off clearing them faster than raising the difficulty. I could graph out the optimal place to run rifts, based on your damage output but there are a few other factors at play. The goal is to clear as fast as possible, unless you can increase the difficulty and still remain below the new clear time. So! What you want to do is get a TTC of 5 minutes for a T1 rift first. Don’t even bother with the other rifts until you can get this one down as it’s very unlikely you’ll get any benefit from harder rifts (exception is playing in groups). Once you have T1 down to an art, then see if a T2 can fit in your time window (7:23). If it can’t, then get better at T1 rifts.
That was a really long post that hopefully added some clarity on the “where should I be farming” discussion. The short answer is “T1 rifts until you can clear them in 5 minutes”. There is very little incentive of moving up in difficulty if you’re aiming for optimal performance.