I don't think there is a clear winner here in what is better. In viewpoint of the promos alone: Blue, Black and Red seem most viable.
The blue promo is really solid, and will definitely be the hardest to spot remove or deal with. Also its inspired ability will help you start to rebound from the huge mana drop and means open mana the next turn for a huge blue creatures (which Theros and BotG have plenty of within the common/uncommon range.)
The black is great for evasion, and itself can spot remove and regenerate itself, however at the expense of more creatures. It also is more susceptible to spot removal (ie; Lash of the Whip plus the usual Hero's Downfall and similar cards) but so is the red promo. Black will have trouble finding good cards to play with, especially when looking at it from a mana curve standpoint.
The red promo is great because it can lock down creatures your opponent(s) control that could block and kill it, allowing you to do a lot of potentially unblocked damage. Also there are plenty of Theros cards that will play well with it, in terms of combat tricks that will protect it from removal and beefy creatures. And red common creatures play well with this dragon.
Green is not too much of a threat in sealed, especially since there will most likely be a few chump blockers on your opponents side. The white promo is simply too expensive for sealed and might not always be bringing good enchantments back. I'd rather play Celestial Archon for its bestow cost. But who says you'll get one. Honestly I'm going to play black, because I enjoy the color, and think I can do well with it.
The blue promo is really solid, and will definitely be the hardest to spot remove or deal with. Also its inspired ability will help you start to rebound from the huge mana drop and means open mana the next turn for a huge blue creatures (which Theros and BotG have plenty of within the common/uncommon range.)
The black is great for evasion, and itself can spot remove and regenerate itself, however at the expense of more creatures. It also is more susceptible to spot removal (ie; Lash of the Whip plus the usual Hero's Downfall and similar cards) but so is the red promo. Black will have trouble finding good cards to play with, especially when looking at it from a mana curve standpoint.
The red promo is great because it can lock down creatures your opponent(s) control that could block and kill it, allowing you to do a lot of potentially unblocked damage. Also there are plenty of Theros cards that will play well with it, in terms of combat tricks that will protect it from removal and beefy creatures. And red common creatures play well with this dragon.
Green is not too much of a threat in sealed, especially since there will most likely be a few chump blockers on your opponents side. The white promo is simply too expensive for sealed and might not always be bringing good enchantments back. I'd rather play Celestial Archon for its bestow cost. But who says you'll get one. Honestly I'm going to play black, because I enjoy the color, and think I can do well with it.