What is the best speed to use when answering a CQ in Morse code?

When responding to a CQ, the golden rule of Morse code etiquette is to never transmit faster than the calling station. If a station is sending at 15 words per minute (wpm), they are implicitly stating that 15 wpm is a comfortable receiving speed for them; answering at 20 wpm might make your signal unreadable to them. Additionally, you must consider your own receiving ability. You should only send as fast as you can comfortably copy, because the other station will likely match your speed for the remainder of the contact. Therefore, the optimal speed is the fastest speed you can reliably copy, but strictly capped at the speed of the original CQ. Distractors suggesting fixed standard speeds (like 5 or 10 wpm) are incorrect because CW speeds are always dynamically adjusted to the operators' abilities.