Vampiric Tutor - The best tutor in the game. Instant speed, 1 cmc, and paying 1/20th of your life is hardly much of a cost for greatness. You can cast it on your upkeep if you need to draw the card the same turn you play the tutor as well.
Demonic Tutor - Tutors for anything to your hand, but is slow. Early on, I usually use Demonic to look for a draw engine or ramp (whichever I'm missing currently). Later into the game, I'll search for a combo piece to win the game, or a removal spell for a pressing threat. I try to not be greedy with my tutors. I don't want them to become card disadvantage, so I like to search for something that draws me more cards if I don't already have a draw engine out.
Enlightened Tutor - I almost always use this to get Necropotence or Phyrexian Arena. Sometimes I want a sac outlet, so I'll grab Ashnod's Altar. A couple of my combo pieces are artifacts, so there are times I search for Nim Deathmantle or Su-Chi to win the game.
Idyllic Tutor - Similar to Enlightened Tutor, but it's slower, costs more mana, and it ca't get artifacts. It puts the card in your hand, but I don't like this card nearly as much as Enlightened because the extra mana AND sorcery can often mean tapping out on mana on my own turn, which means people don't need to fear the excellent removal suite that Orzhov has to offer.
Necropotence - This is my absolute favorite card of all time. It ensures that my hand is always full. When I feel like I need to be conservative with my life total, I use Necropotence to refill my hand on each of my turns. If I have out Reliquary Tower, I have no problem paying huge amounts to dig for win conditions. This deck has so many creatures with Lifelink (including the commander), so I tend to gain back the life I spent to draw cards.
When I think about Necropotence, I conclude that it is card advantage incarnate. Think about it. Cast one card, pay , draw as many cards as possible. What more could you want? I try to get black mana out as soon as possible, so that I can start drawin' them cards.
The only time I've found Necropotence to be problematic is after I've paid a bajillion life to draw half my library, then someone at the table wheels my hand, which then gets exiled. Or when I have a massive value creature or win in my hand and someone forces me to discard. I err on the side of caution if someone's playing a commander which tends to wheel a lot like Leovold, Emissary of Trest, Nekusar, the Mindrazer, or Nicol Bolas. I'm also weary of anyone who casts Waste Not, since that person will (obviously) force you to discard a lot, and since Necropotence's exile clause is a trigger, NOT a replacement effect, your opponent will still get all of his or her Waste Not triggers. I will also note that for this same reason, if someone uses Windfall, your hand is still counted for how many cards everyone will draw.
Phyrexian Arena - The much lesser, much less potent babby nephew of Necropotence. As bad as that sounds, Phyrexian Arena is still an extremely powerful card. Drawing 2 cards per turn at the cost of 1/40th of my life total? Sounds good. Something nice about Phyrexian Arena is that it doesn't draw as much of a target on my head as Necropotence does, so I can draw 10+ cards off of it before the end of a game, since it tends to not get removed.
Dark Confidant - Yet another life paying card draw engine. Dark Confidant is more easily recurred from a graveyard than Necropotence or Phyrexian Arena because it's a creature. It's also nice that when I'm in dire straits, I can sacrifice Confidant to one of the many sac outlets in my deck.