Hot Picks: So You Want to Worship Hell...
Okay, let's summarize. Which of these Patrons and Paths are worth your consideration? This is the TLDR post for those of you who don't want to read through the 16 individual posts in the thread above. I'm breaking these into two groups: General Interest, and High Level Only, for those that don't give you anything great until the third and final Boon.
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General Interest
Baalzebul's Evangelist Path gives you a +20 untyped bonus to Intimidate checks at 13th level. That's very solid, and worth building towards. Casters aren't usually Intimidate monkeys, but you don't even have to be a Cha-based caster to take advantage of this -- just taking a trait or throwing a few ranks at Intimidate will let you use it regularly. It doesn't hurt that Baalzebul has a good benefit (+4 against mind-affecting effects) to tide you over while you wait to reach 13th.
Barbatos has a decent benefit, and his Sentinel Path gives you free Glyphs of Warding followed by a free Gate spell at 16th level. You can use the Gate either for planar travel or to call a pit fiend, and you don't have to pay the up-front 10,000 gp cost. You can't control the fiend, but it's RAW that it will "tend to gladly obey". This is a bit blurry, but unless your DM is a real bastard this is pretty solid.
Belial, at 13th level, lets you change yourself into any sort of outsider. You then gain that outsider's immunities, resistances, and Fly speed. This is just excellent, and kind of abusable in a good way. There are outsiders that are immune to pretty much everything. Like, have you looked at the Inevitable subtype? Those guys are "immune to death effects, disease, mind-affecting effects, necromancy effects, paralysis, poison, sleep, stun, and any effect that requires a Fortitude save (unless the effect also works on objects, or is harmless)." The kami subtype is almost as bad: "bleed, mind-affecting effects, petrification, and polymorph effects" plus Resist acid 10, electricity 10, fire 10. (And, yup, it says "any outsider" , so native outsiders like the kami are totally on the table.) And you can *swap around* your chosen outsider three times per day. If you have one standard action of warning, you can make yourself immune to everything from critical hits to energy drain. This is a fine all-round defensive ability, and that's before we consider the fun RP aspects of pretending to be an angel or whatever. Getting a Fly speed on top is just gravy.
The Archduke Jiraviddain has a slightly unpleasant Obedience and a junk benefit, but then two good spells -- Death Knell and Heal, with the latter being particularly nice to have if you don't have a healbot in the party -- and then Symbol of Vulnerability. That's normally a very meh 9th level spell, but (1) it's another "SLA, so no component, so you save a bunch of money" -- in this case, 15,000 gp per casting; and (2) it's particularly useful for a Diabolist, since it melts away creatures' defenses without allowing a save or Spell Resistance. A high-level character with access to this SLA would literally paint his living quarters with these things.
The Archduke Lorthact has an easy Obedience and his benefit is a +4 profane bonus on Knowledge and Spellcraft checks. That is ALL Knowledge and Spellcraft checks. There are 10 Knowledges, so that is like getting 44 skill ranks, boom, free. If you're a Knowledge monkey, this is very hard to resist.
Mahathallah's benefit (which you get as soon as you take the Obedience feat, as early as 3rd level) gives you +2 on the DC of all your illusion spells and a +2 bonus on saves against mind-affecting effects. That's 2.5 feats for the price of one! If you're playing an illusion caster, and you don't mind being LE, go worship Mahathallah and take this feat right now. Mahathallah's Boons are nothing special -- the Sentinel path in particular is amazingly bad -- but you could do something with the Evangelist and Exalted if you want to build a character around illusions and drugs. Honestly, though, she's the go-to deity for characters who don't much care about the boons: to wit, illusion specialists who just want that sweet, sweet free feat and a half early on.
The Archduke Malthus' benefit is "a +4 profane bonus on Reflex saving throws to avoid physical dangers." Pretty much all Reflex saves are to avoid physical dangers. So, this is basically +4 on Reflex saves, or the equivalent of two feats.
Mammon's Sentinel path gives you the power at 13th level to shut down Smites and other good-based damage. This is situational but very thematic, and you could definitely build a fun NPC with it. "Sir Bluto Saunce Pite has killed a dozen paladins. We're not sure how, but..."
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High Level Only
Ardad Lili's Evangelist Path gives you Soul Bind as an SLA, meaning no material component cost. Savings: potentially huge! The Archduke Deumas also gives you this, although with a nastier Obedience.
Doloras' third Sentinel Boon gives you a burst attack with a 30' radius, centered anywhere within 100'... and creatures in that radius are seized by hellish chains and immobilized for rounds/level, and can't fly or teleport way. If a target makes a Reflex save, it is STILL immobilized for half the duration. And if it fails, it takes "2d6 points of piercing damage and 4d6 points of fire damage per round as blazing, hellish needles stab into the exposed portions of their bodies". No damage cap. So, if a creature doesn't have fire immunity or resistance, it will take 6d6 of damage times your level, which at 16th level is 336 points of damage. That's on top of all the damage you can pour into it while it's immobilized. Hell, even if it makes its save, "immobilized for 8 rounds" probably means it's dead anyway. This is a very powerful tactical suck-or-suck. If you throw it at your PCs, try to give them some advance warning first...
Geryon's final Exalted boon lets you completely shut down enemy faith-based casters, simply cancelling the ability of clerics and oracles to cast spells. It's Cha-based, so -- given that it allows a Will save, and clerics tend to have great Will saves -- you really need to be a Cha-based caster to pull it off. But you could make some cool NPCs with it! I'm imagining a devil-themed sorceror who calls himself "the Faithbreaker"...
And finally, the Archduke Lorcan has a yucky Obedience and a mediocre benefit, but is the only Diabolical Patron who gives you access to True Resurrection. As a SLA. Which means no material component cost. That means a massive 25,000 gp savings every time you cast this spell. You can cast True Resurrection for free every darn day. Even at levels 16+, this is so good as to verge on unbalanced. Why Hell allowed a relatively lowly Archduke to have near absolute power over life and death is unclear, but there it is.
