A fixed voltage battery is typically around 3.7 volts. The best resistance is either a 1.5 or 1.8 ohm coil. My preference is 1.8 ohms for fruit flavors (they like a slightly cooler vape) and 1.5 ohm for bakery flavors (they prefer a bit more heat). A 2.1 ohm coil is probably more resistance than you really want with only 3.7v to use.
When you get a chance, get yourself something like a Vision Spinner or Spinner II that has variable voltage from 3.3v to 4.8v. With mine, I just use 1.8 ohm coils all the time (for my tanks), and I can adjust the voltage to the juice I'm vaping at the time. When I direct drip with a 2.0 ohm HH.357 atomizer, I set it at 4.3v. With a tank and 2.1 ohm coil, I go with around 4.0v for optimum performance -- for me. YMMV.