Which wood should I use for my CNC project? Species behavior, grain, and what carves well

The rule of thumb

Tight, even grain carves clean; soft/uneven grain fuzzes and chips. Density buys detail.

Maple (hard): the v-carving gold standard, with crisp edges and fine detail. Watch for burning (see the burning entry): sharp bits and honest feeds.
Cherry: beautiful, carves crisply, burns the easiest of the common woods; darkens with light over time.
Walnut: excellent carver, forgiving, hides small flaws in its color; brittle across short grain in fine details.
Oak (red/white): carves fine but the coarse open grain visually interrupts fine detail and v-carved lettering, so it is better for larger/bolder designs. White oak > red for weather.
Ash / hickory: tough, carve OK, hard on bits; hickory chips at fine detail.
Poplar: cheap practice hardwood; carves decently, slightly fuzzy, paints well (greenish color argues for paint).
Pine / SPF construction lumber: hardest to carve WELL. Alternating soft/hard growth rings tear and fuzz, knots wander, sap gums bits. Fine for practice and rustic looks; use sharp bits, shallow passes, and seal punky spots (50/50 PVA/water) before carving. Yes, the cheap wood is the difficult one.
Cedar: soft, aromatic, rot-resistant (outdoor signs); fuzzes like pine, splinters in thin detail.
Basswood / butternut: traditional carving woods, soft but even. Lovely for 3D relief, too soft for crisp shallow v-carving.
Exotics (padauk, purpleheart, wenge...): dense, stunning inlays; some (wenge, cocobolo) are splintery and their dust is extra-irritating, so wear a respirator.

Fine v-carve detail suffers where the design crosses grain sharply, so orient the design so critical fine elements run WITH the grain when possible.
3D finishing raster along the grain gives the cleanest look on the final pass.

Glued panels: match species AND grain direction across the glue-up or the carve reveals every joint.
Acclimate lumber in the shop (see the slab-flattening entry).