If you only have two minutes before pairings go up: Diana and Master Yi are the safest choices, Kha'Zix improves most with the right upgrades, Ivern needs tags, Vi needs more ways to draw threats, and Jhin needs you to open the right spells.
For the full card-by-card explanation, read the companion breakdown: Unleashed Pre-Rift: Which Seeded Legend Should You Pick?
Diana
Strong answers + easy third domain
You can find the cards you need, answer enemy units and spells, and play a deck that works without many upgrades from your packs.
Master Yi
Gets stronger as the game goes long
Yi gains XP reliably. At Level 11, every unit enters ready, making it much easier to fight for several battlefields in one turn.
Kha'Zix
Spend XP for immediate pressure
The starter deck already supports its main plan, but Scryer's Bloom is slow, Call to Battle invites opponents to your battlefield, and Ripper's Bay does nothing for this deck. Every strong card that rewards XP makes the Voidreaver plan scarier.
Ivern
Needs enough animal-tagged units
Brush matters only when you have Bird, Cat, Dog, Poro, or Ivern units that benefit from it.
Vi
Fast attacks with awkward triggers
The good draws hit hard. The problem is sustaining pressure while still setting up 3+ excess damage.
Jhin
4+ energy spell puzzle
The cards are playable. The legend asks for more 4+ energy spells than the seeded kit naturally provides.
Quick recommendation
Safest picks
Diana or Master Yi. Do not overthink it. Both decks already know what they are doing.
Depends on your packs
Ivern and Kha'Zix both get much better when your packs contain the right support. Count tags and cards that reward XP before finalizing your deck.
Danger zone
Vi and Jhin can still win games, but their legends ask you to solve a real deckbuilding problem first.
See you on the rift,
Shadow