We'll begin the review of the Blood Angels Codex from a 6th edition standpoint with the HQ choices. Blood Angels have some funky FOC placements, and in HQs mean you can't take a Chaplain here (but a tad more expensive Reclusiarch). Not terribly bad though. After each HQ choice I'm also commenting their related special characters, if any.
The top contenders:
Mephiston: While the other special characters were essentially buffed / personalized versions of the normal HQ, Mephiston is a completely different animal from a Librarian. The best way to portrait him is as a MC in power armour. His small size means he can be easily hidden behind / inside razorbacks or any other transport. He lacks an invulnerable save but with a Priest around he can take FNP against pretty much anything as far as I know: Vindicator shells, lances, earthshaker rounds, you name it... and of course the ubiquotous plasma that is meant to be his nemesis. His sword is AP3 so be mindful of whom you're throwing him against; Mephiston is probably at his best as a beater unit of scoring units.
The situational choices:
Astorath: Astroboy comes with a peculiar set of special rules, which you pay for, and thus make him a very specific role. As a buffed Reclusiarch comes with 2+/4++ and a S6 AP2 weapon that strikes at I1 and causes to reroll any invulnerable saves passed against it. All in all he is a pretty good challenger. He also unlocks the 0-1 Death Company limit, which shouldn't matter much given they are not scoring. 220 points is a steep price though and most times there will be better options around.
Dante: The Chapter Master of the Blood Angels is still a usable character despite a somewhat rough transition to 6th. On one hand he retains his main abilities - Hit&Run is useful as always and a strange sight in Marine armies, and no dispersion when Deep Striking with arriving with his jump pack. However he now wastes his I6 by using a Power Axe, and his DS abilities won't see that much use now that Descent Of Angels style of armies are not possible. Yes you can attach him to a buffed unit and DS exactly where you want them to be, but part of your army will still be moving out of your DZ and dropping +450 points with little support... is risky.
Honor Guard: While not a proper HQ, they can be unlocked by any HQ choice so we'll cover them here. They are essentially 5 veterans with an embedded Sanguinary Priest that can be kitted for a shooty or combat role, at a hefty price tag. So you can have 4 special weapons and a priest in just one unit, or perhaps some powerfists / energy weapons / storm shiels. Either case, great versatility but the price goes up swiftly. They will be very high in the opponent's priority list so consider well if you need them, and how much can you afford to invest.
Best leave at home:
Captain Tycho: Tycho comes in 2 flavours. The golden one is a buffed captain, sporting a shiny artificer armour (yikes that's a 2+), a combimelta with special issue ammo and an armour ignoring weapon that strikes at normal Initiative (!). As normal IC he can join any unit he wants and will bring some utility. Sadly 175 points is a steep price for what he brings. His best use is probably leading Sternguard Vets.
The dark version is after Tycho's inclusion to the Death Company, so his statline is slightly altered to favor CC instead of shooting, and gains the normal DC rules (relentless, rage, furious charge, FNP). Now this would be interesting if it was because Tycho can-not-join any unit, not even the DC. What's he good for then? I haven't a clue.
The Sanguinor: He is truly a relic of the past, in all the meanings of the word. As far as 6th edition is concerned, the Sanguinor is a character killer that buffs a sergeant and has a small +1A aura. But 275 points for a 3W / T4 character, even with 2+/3++, is excessive.
Mephiston is a great choice in hybrid armies where he can hide behind razorbacks, and is a real pain to deal with for many armies. Dante has its place in DOA heavy lists, and a Reclusiarch gives you a chaplain and fills your HQ quota, if that's what you want. The rest of options are better left at home, either because of their price tag (Sanguinor) or because they are objectively worst than others (Captain).
Stay tuned for the Elites review coming soon!