Ser Dontos Hollard
Why it could be him:
- In the premiere, Ser Dontos approached Sansa on the castle grounds, thanking her for saving his life at King Joffrey's name-day tournament, where he showed up drunk. He also gives her a family heirloom necklace as a token of his appreciation, saying, "Once I was a knight, and now I'm only a fool. I may be a fool, but I'm a living fool thanks to you."
- Right after Joffrey dies, Ser Dontos arrives at Sansa's side, urging her to come with him if she wants to live.
Why it couldn't be him:
- When Ser Dontos performs at the wedding, Joffrey offers a gold dragon to whoever knocks his hat off, encouraging people to throw things at him. Ser Dontos leaves after that, though, so it isn't likely he directly poisoned the king.
- Pretty sure we can count Ser Dontos out, given that Littlefinger has him killed after delivering Sansa. The necklace bit was also part of Littlefinger's scheme; Dontos was clearly just a pawn.