First you need to pick the right color. Since you can use the promo is gonna be one of your 40 cards.
The top promos for me are:
1. Dawnbringer Charioteers
First it's the fastest drop, it's a 2/4 flying, lifelink and the Heroic ability can boost your creature on the early game. White has a few first drops and removals.
2. Heroes' Bane
For five a 4/4 that can double the counters it's a good limited card. In the late game can cause the opponent a lot of troubles and if you can open some of the green accelerators like sylvan Caryatid
Or golden hind
Can cause trouble early on the game
3. Scourge of Fleets
For the third spot whe have mix feelings but thinking about advantage on the late game this card could be a key victory and awkward surprise for your opponent. Everything is on the details, if you don't have a lot of islands, you can't return the big creatures that you whant to get out of the way.
4. Spawn of Thraxes
Red has a lot of good 1-2-3 drops but these promo it's not that good, unless you don have the luck to open the burns and the top creatures these promo it's useless.
5. Doomwake Giant
For us these is the least playable card on limited because you need a lot of mana and resource to take advantage of the ability.
Now that you pick your color you get to open your boosters and if you are lucky you open one of these; consider if it's good to splash another color. I know is hard to force a second or third color if the cards of the splash color are not strong but these cards could win a prerelease.
