It mostly depends on the board, how much mana you have access to, and how you think the next few turns of the game will go
On top of Afflict, Lazotep Sliver also lets you Amass Slivers 2 whenever another one dies
Pecharunt ex can be paired with Okidogi ex, Munkidori ex and Fezandipiti ex to unlock different play combinations
Theres just something powerful, mysterious, and scary about creatures that change form
Mega Starmie exs damage-spreading capabilities are especially popular in tournaments, Mega Zygarde ex has a ton of Fighting-type support, Mega Dragonite ex boasts one of the most powerful attacks in the game, and Mega Skarmory ex can threaten key support Pokmon, such as Meowth ex
So if you want some for your collection or are looking for a good cheap investment, the enemy buddylands are where it's at on Magic Online