Only play QoT, UB, and Erebus. Always go Cloak of Night with Queen and Blood with Erebus.
I used to go Blood with UB, but have recently been playing with Dark Crimson Vial (DCV). This is quite a nice item. It gives 33% of your max health upon usage. Though that is less than a sigil (which gives 50%), it is like having a permanent sigil without using a consumable or inventory slot. In some ways, it's better. For example, the hitpoint boost is permanent, and not just for a number of seconds. Cast time is just one second, and I have yet to be either killed or interrupted while using it. The cooldown time is 45 seconds, which seems reasonable given the strength of the item. You'll need to be careful at the beginning of the game, since you'll have less base health than those who take Blood. However, as you stack health, the DCV really shines. Combine it with sigils and you can almost completely replenish your health in a couple seconds (33%+50%).
For example, let's say you have a demigod with max health of 6000 who gets reduced to 1000 in combat. Using a sigil adds 3000 (or 50% of 6000). Using DCV adds another 2000 (or 33% of 6000). Viola! You are back up to 6000 health, and 2000 of that is added permanently.
Disclaimer: I am not saying that DCV is a replacement for sigils. Indeed, it works best when used with sigils. Also, DCV works well for players who are patient and cautious at the beginning of the game, but plan to have a lot of health by mid-game. The value of any favor items should be based on whether it enhances your survivability and keeps you in the field longer (that is, reduces the number of trips to the shop/crystal). DCV is very good at doing both, and its effect scales well over time.