There's been a lot of debate on the blog as of late of which is the best choice for the heavily contested Daemon Elite slot. With a bunch of mathhammer, I am going to solve the problem.
Consider this an addition to the Elites review.
Beasts of Nurgle are out.
"doesn't anyone love me?"
We'll use closely points-equivalent squads of each-
210pts, 7 Fiends. 
205pts, 5 Flamers with Bolt. 210 pts,
200pts, 5Bloodcrushers.
In each chart, the green cel is the one that did the best.

First we'll look at their anti-infantry: how many GEQ, MEQ, TEQ and carniFEX they kill on the charge (or flame, assuming flaming hits 4 models each)
I checked warpfire for the flamers, it's worse than Breath.



GEQMEQTEQFEX
Flamers10106.662.5
Fiends17.1174.084.66
Bloodcrushers11.1111.117.4076.66


It's slightly surprising that Bloodcrushers are better at killing MEQ and TEQ than flamers- only by one model, mind you, but it is surprising. Fiends are far and away the best at killing GEQ, but the lack of power weapons seriously hampers their ability to kill tougher opponents.

Now let's look at Anti-tank. One flamer will use Bolt, the rest will use Breath. There will be 3 separate charts, for stationary, combat speed and cruising speed, and each chart will have the full range of AV's. P for penetrating, G for Glancing.



StationaryAV10AV11AV12AV13AV14
FlamersP0.55, G3.11P0.44,G3.11P0.33, G3.11P0.11, G3.11G3.11
FiendsP7, G7P7P4.7, G2.3P2.3, G4.7G7
BloodcrushersP6.66, G3.33P3.33, G3.33G3.33--


CombatAV10AV11AV12AV13AV14
FlamersP0.55, G3.11P0.44,G3.11P0.33, G3.11P0.11, G3.11G3.11
FiendsP3.5, G3.5P3.5P2.35, G1.15P1.15, G2.35G3.5
BloodcrushersP3.33, G1.66P1.66, G1.66G1.66--



CruisingAV10AV11AV12AV13AV14
FlamersP0.55, G3.11P0.44,G3.11P0.33, G3.11P0.11, G3.11G3.11
FiendsP1.16, G1.16P1.16P0.78, G0.38P0.38, G0.78G1.16
BloodcrushersP1.11, G0.55P0.55, G0.55G0.55--


The Fiends are the best at killing stationary and combat speed vehicles, hands-down, but the Flamer's consistent 3 glancing hits gives them an edge over the Fiends at cruising speed. So, I suppose if you really want to immobilize a rapidly approaching land raider, use flamers, but for everything else? Fiends.


Next is durability, namely, how many boltgun rounds, S4 power weapons and S6 power weapons, and meltaguns it takes to kill the entire squad.



BoltS4 powerS6 powerMeltagun
5 Flamers20201212
7 Fiends424225.225.2
5 Crushers904522.518



The Bloodcrusher's high toughness is their primary defense, more so than their armor save. This shows when they perform better in the face of S4 power weapons, but when their high toughness is bypassed, the Fiend's weight of numbers shows through.


Well, this whole thing shows pretty well what we already know. Flamers are meh at everything. Fiends' low cost lets them be durable tank hunters and guardsmen killers, and Bloodcrushers can tear through almost any non-vehicle, and absorb silly amounts of anti-infantry firepower.