Hello! After reading this great thread and observing the success of this deck on Magic Online, I decided to take a version of this list to my local Legacy tournament today. In a 53 person tournament, I went 5-1 to make the top 8, but then lost in the top 8. My list was this (copied exactly from one of the mtggoldfish lists):

4 Griselbrand
4 Sire of Insanity
1 Iona, Shield of Emeria
1 Blazing Archon

4 Lotus Petal
4 Dark Ritual
4 Entomb
4 Reanimate
4 Exhume
4 Animate Dead
4 Thoughtseize
4 Unmask
4 Faithless Looting

2 Marsh Flats
2 Verdant Catacombs
2 Polluted Delta
2 Bloodstained Mire
3 Badlands
1 Bayou
2 Swamp

Sideboard:
4 Abrupt Decay
4 Stronghold Gambit
3 Faerie Macabre
2 Pithing Needle
1 Inkwell Leviathan
1 Tidespout Tyrant

I went up against a slew of combo matchups that Reanimator typically feasts on and destroyed almost all of them. Beat Infect, beat Doomsday, lost to MUD, beat Soldier Stompy, beat an Affinity brew, and beat Goblins. Then lost in the top 8 to BUG Delver.

My thoughts on my list:

I would not change a single card in the maindeck. I understand the merits of running a fourth Badlands instead of the second basic Swamp, but there was at least one situation today when having the second basic saved me. My Soldier Stompy opponent had a new Thalia out and upon activating my Griselbrand, I needed to draw into my one remaining Swamp in order to have enough mana to Abrupt Decay his Chalice, then chain Rituals into a reanimated Blazing Archon. Luckily I drew into it, but had I not, I would have been one mana short and would have lost on his swing the following turn. Maybe this is just one obscure corner case, but I do like the relief of having a second basic in general with this deck, the reasons already mentioned in this thread.

Blazing Archon is not always included maindeck, but I liked having him as the emergency option to best keep me alive in a rough spot. Since several decks simply cannot win at all once he hits the table, he gets a maindeck slot (being immediately game-winning is exactly what you want from most of your creatures). A creature that is untargetable or can remove permanents seems more situational in matchups where those are wanted, so I decided to keep those in the sideboard.

I have several doubts about my sideboard, however.

- Abrupt Decay was the MVP of the sideboard. It was amazing in every matchup that I brought it in, and it came in almost every time. I would play more than four if allowed.

- Stronghold Gambit worked well in the matchup (Doomsday) that I brought it in against, but that was the only matchup I used it and I'm not even sure it was really necessary. I do like the idea of including four copies of sideboard cards, to go all-in on one type of sideboarded answer. So maybe a restrictive Show and Tell is the best type of answer (over removal) for enough decks to warrant some number of Stronghold Gambits. I am really on the fence about this card, but will probably try four again simply because it's so fun and gratifying to finally get that card to work. Also, our Thoughtseizes and Unmasks clear the way very effectively and make Gambit safer for us to use than probably any other deck.

- Faerie Macabre didn't get used once today, but they are probably necessary. I will keep them.

- Pithing Needle was easily the worst sideboard card today. Even in the matchups like Goblins and BUG Delver where I could have brought them in, I opted not to. I used to play UB Reanimator almost exclusively, and that version of Reanimator seemed more about "finesse" and tip-toeing around an opponent's hate. Pithing Needle seemed more necessary in that version, to stop answers like Maze of Ith and Karakas and make Griselbrand better. But this red version seems more about brute force, and rushing out a Sire or Griselbrand as fast as possible. Just getting the creature out quickly should win the game by itself, and I think the hate should just be about working to that end. Needle seemed a lot worse at accomplishing what Abrupt Decay did better. I will replace them for the next tournament.

- Inkwell Leviathan and Tidespout Tyrant never seemed necessary for me to side in, but I think every Reanimator should have a creature that functions the way each of them do. They will stay.

Where I go from here:

The first thing I want to try out is replacing the two Pithing Needles with two Engineered Explosives. Has anyone tried Explosives in this deck? I think the card could be like Abrupt Decays 5-6, serving as removal yet being a little different. It answers almost every graveyard hate card (sans Leyline) plus it can potentially sweep multiple Deathrites and Delvers in one go. I always play Explosives in 12Post (my primary deck) and I love it. And it seems like it would have even more use with Renimator.

I might try to squeeze in 3-4 copies of Reverent Silence, but I question whether that much sideboard space is really needed to fight Leyline. For players that run them in the sideboard, do they feel absolutely necessary? Also, is the requirement of having a Forest in play ever a problem? Unlike Abrupt Decay, we need the Bayou in play and can't rely on Lotus Petal (unless we're paying the full four mana). I question whether that much sideboard space is required for a card that seems kind of narrow.

Also, I like the idea of Chancellor of the Annex strengthening our early game, but I don't know about how much space gets taken up by four copies of the card. I've noticed that most builds that run four Chancellors eschew some number of Sire of Insanity, and I don't like cutting the Sires. I think the full set of Sires plays very well into the speed of this deck (when you can get a big creature first turn, Sire almost always seems like the best choice), plus they increase your black card count to help cast Unmask. Also, hardcasting Sire is very possible with the Petals and Rituals, which is a pretty unique angle of getting a big creature into play. And Chancellor doesn't seem spectacular as a reanimated creature against several opponents.

In conclusion, I was astounded by the sheer speed, power and consistency of the deck! This thread has been incredibly useful, and I will be happy to contribute to it!