I'll use whatever's handy, but since a pie is usually in a glass or metal pan, I won't use a sharp knife (like a chef or slicing knife). I don't ever use that kind of cutlery on plates or hard surfaces. Probably my favorite thing for pie is a palette knife ... not even a real knife. But super thin, and it can then be used as a spatula to serve the slice. They're useful for a million other things; I always keep a straight one and an offset one around. And they cost around $5
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image by Ateco USA