I would say either Tara or Giles. Tara was so angry she broke up with her, understandable because Willow was playing dangerous games with her mind, and her addiction was getting very dangerous for their relationship.

Giles would be second I think, because he's been there before, when he was Ripper. He would probably be angry at her and his own inability to see she needed some training and to learn control.

Buffy, Xander, Dawn, I don't think they realized the full extent of what Willow was going through at the time or how dangerous she could become. Spike doesn't like magic, so he might have been concerned a little if he was close to Willow, but he wasn't.

I would say Giles, "you rank, arrogant amateur!"

Giles knew the consequences. Even more than Tara, I think, having been there himself. Which probably also explains his compassion afterwards. But Tara was more emotionally hurt, I believe. Dawn was only angry at the personal damage to herself.
Buffy and Xander never get it. Spike no like the magic, but at least he understand the consequences.

Gile's was pretty angry!!But Tara was much closer to her so it would of had a bigger effect!!Tara was just so hurt,but Gile's did know willow longer& was like her teacher!:)

nika, i think spike was mad because he was worried about buffy but i would still say giles

Yeah, Spike was probably mad at her because he was worried about Buffy and Dawn now that I think of it. Your right.

Tara understood the most. She not only knew about magic and its consequences, but also, she was closest to Willow and understood better than anyone what it would do to her.

Giles is a pretty close second, but even though he had experienced his own "darkness" in the past, he sort of stood apart a little...that is, I felt like even though he cared for Willow, he was angry at her almost objectively, because she was becoming a big player for the bad side. Not so much because she was destroying herself.

I think Tara was more worried than angry. I don't think she left out of anger, but more out of hurt and kind of as a wake-up call to Willow.
The fact that Giles confronted Willow showed that he was extremely angry because he has always chosen his battles carefully. He keeps his peace unless he really thinks something needs to be said.

i agree with xElvenPiratex tara was more worried then angry

Giles was more angry, Tara was worried and disapointed.