2 x Board
The common 2x board is the most commonly used floor and ceiling joist because of it's easy availability and low cost.
In this photo, 2x8s are used as floor joists, nailed to a heavier laminated beam (used to carry loads over longer spans).
2x boards work just fine as floor joists.
But again, beware of using warped ones (more likely the longer the piece is).
2x joists are also the most likely to produce annoying floor squeaks, which unfortunately, are often only discovered after the house is finished and settles a bit.
What is called 2 inch width is actually only 1 ½ “. Planing and shrinking of the board in the milling process account for this (that’s what they claim, anyway).
That’s an easy target to miss with a nail, especially when shooting through ¾ “ of plywood subfloor.
Any area of the subfloor that is not tightly fastened to the joist beneath can give way slightly underfoot. That is where squeaks come from.