I think Arkn (R) is the best 1 pill card in the game. Especially when you consider for 2 stars what he can do. -10 attack. Most of the time people play 1 or no pills against your Arkn (R) giving you the win for 1 pill. Alternatively they play 4+++ pills, and you get the pill advantage. I usually lead with Arkn (R) when possible. Additionally - you're 1 pilling a mediocre card, 1 pilling a card you have to win is sometimes risky. Arkn (R) isn't risky ever.
What are your favorite 1 pill cards?