Whatever fits within your budget easier, really. Thigh-high boots can run a little, well. High. You probably won't be able to completely bridge the height gap between yourself and your Lelouch, but you might consider some platforms.
Depending on how focused you are on detail, I might recommend using boot covers, because they're easier to just applique the gold bits onto, and you'd be able to get that nice sturdy point at the top of the thigh. If that amount of detail isn't a priority, then standard stretchy thigh-highs might be fine (with the gold parts painted on)