Taiga

a Mountain enters the battlefield — 64 cards in the game can cause this (Taiga is one).

1. Last Light of Durin's Day — When Taiga enters, Last Light of Durin's Day puts a quest counter on itself
2. Staff of the Flame Magus — When Taiga enters, Staff of the Flame Magus gains you 1 life
3. Valakut, the Molten Pinnacle — When Taiga enters, Valakut, the Molten Pinnacle deals 3 damage

a Forest enters the battlefield — 85 cards in the game can cause this (Taiga is one).

1. Titania, Nature's Force — When Taiga enters, Titania, Nature's Force makes a token
2. Baru, Fist of Krosa — When Taiga enters, Baru, Fist of Krosa gives +1/+1
3. Reach of Branches — When Taiga enters, Reach of Branches returns itself from the graveyard
4. Tourach's Chant — When Taiga enters, Tourach's Chant deals 3 damage
5. Battlewand Oak — When Taiga enters, Battlewand Oak gets +2/+2
6. Staff of the Wild Magus — When Taiga enters, Staff of the Wild Magus gains you 1 life
7. Necklace of Girion — When Taiga enters, Necklace of Girion puts a counter on a creature
8. Roiling Canopy — When Taiga enters, Roiling Canopy gives +3/+3

a land enters the battlefield — 1,542 cards in the game can cause this (Taiga is one).

1. Primeval Bounty — When Taiga enters, Primeval Bounty gains you 3 life
2. Dancing from Dark to Dawn — When Taiga enters, Dancing from Dark to Dawn makes a token
3. Toph, Earthbending Master — When Taiga enters, Toph, Earthbending Master puts a counter on a permanent
4. Emeria Shepherd — When Taiga enters, Emeria Shepherd brings a card back
5. Quest for the Necropolis — When Taiga enters, Quest for the Necropolis puts a quest counter on itself
6. Soul Stair Expedition — When Taiga enters, Soul Stair Expedition puts a quest counter on itself
7. Nissa of Shadowed Boughs — When Taiga enters, Nissa of Shadowed Boughs puts counters on a permanent
8. Thranduil, Sindarin Liege // Silvan Rally — When Taiga enters, Thranduil, Sindarin Liege // Silvan Rally makes a token
9. Silvan Reveler — When Taiga enters, Silvan Reveler returns itself from the graveyard
10. Floral Evoker — When Taiga enters, Floral Evoker puts a +1/+1 counter on itself

190 other cards care about it too. These are the ones with the most connections.