When ATC says "cleared for takeoff", what's the correct answer from the pilot?
This is how FAA recommends (PDF):
A typical takeoff clearance may state, for example, “(Callsign) 123 RNAV to MPASS, Runway 26L, Cleared for Takeoff”.
The expected pilot response is, “(Callsign) 123, RNAV to MPASS, Runway 26L, Cleared for Takeoff”.
Same phraseology you're given: cleared for takeoff with your callsign, and if applicable, the runway or instructions ("turn left heading 200" for instance) given in the same transmission. Anything else is nonstandard.
Not on the roll nor on the go nor cleared for departure nor anything else, no matter how cool it sounded when somebody else said it on the radio.
Quite simply, you readback the runway identifier and the clearance - exactly like you receive it.
In case you get other instructions in connection with the takeoff clearance, you need to read those back as well.
Note that you should not read back any wind information given, since it is just that - an information - not a clearance or instruction.
ICAO Document 9432, section 4.5