A bullet with a twist dial, 4/5 rumbly, low pitched vrooms and infinite lifespan.

If I wasn't allowed other toys as well then: something that I could use as a clit vibe or an aggressive g-spotter (like my Amethyst). I think Lelo Gigi or Ophoria Beyond 1 would fit the bill although I haven't tried them. Rabbits can't be forced to press against my g-spot hard enough even when they are curved. So I would trade having to do clit stim by hand during g-spot play.