Whichever combo fits your budget and maximizes the time you want to spend on the bike acquiring fitness. Sure, a $200 Walmart bike can help accrue fitness. But you will likely hate the bike and not want to ride it much.
If you buy a pro road race bike and can't handle the aggressive fit, might not want to ride that much either.
Also need a bike that fits your terrain. Having a bad mismatch on your local riding terrain to the bike you have will make you not want to ride it as much.
But otherwise, this is the troll topic about "I can get more fit on a heavier bike" and "put a car battery on my bike to make the hills longer" nonsense.