A bathroom floor gets wet, repeatedly, in small amounts, for decades. Everything else about the choice follows from that.
Porcelain tile — the default
Impervious, durable, available in every appearance including convincing wood and stone looks, and it tolerates standing water indefinitely.
Cold underfoot, which electric underfloor heating solves at modest cost during a remodel and at considerable cost afterwards.
Hard, so dropped items break and standing for long periods is less comfortable.
Grout needs sealing on a cement grout, or use an epoxy grout which does not.
This is the right answer for most bathrooms and it is the reason it is the default.
Luxury vinyl plank
Waterproof through the body of the plank, warmer and softer underfoot than tile, quieter, and considerably cheaper to install.
Check the core. Rigid cores — SPC and WPC — are genuinely waterproof. Anything with a wood or paper composite core is not, whatever the marketing says.
Seams matter. A click-lock floating floor has seams that water can reach in a room where water reaches the floor regularly. Glue-down products remove that concern.
Heavy fixtures — a freestanding tub, a vanity — sit on a floating floor and restrict its movement, which is a real installation consideration.
Natural stone
Beautiful and demanding. Porous, so it needs sealing on a schedule, and some stones etch on contact with acidic cleaners.
Marble in a bathroom will develop a patina, which some people want and others read as damage. It is a decision to make deliberately.
What to avoid
Solid hardwood — it will cup and the finish will fail at the seams.
Laminate — the core is compressed fibreboard and it swells irreversibly.
Carpet, for reasons that need no elaboration.
Slip resistance
The number is a coefficient of friction, and manufacturers publish it. A rating of 0.42 or higher wet is a widely cited threshold for floors, and higher again for a shower floor.
Smaller tiles in a shower give more grout lines, which is the practical source of grip.
A large polished porcelain tile is dangerous when wet regardless of its rating, and it is chosen for appearance frequently enough to be worth stating plainly.
What is underneath matters more
Deflection. A floor that flexes cracks tile and opens grout. The industry standard for tile is a deflection limit of L/360, and for natural stone L/720 — twice as stiff. Where the joists do not meet it, the floor is stiffened before tiling, not after.
Uncoupling membrane. A layer between the substrate and the tile that lets small movements happen without transferring them to the tile. Cheap insurance on any tiled floor and close to essential over a wood subfloor.
Waterproofing at the shower, extending out onto the floor.
Underfloor heating, which has to be decided before the floor goes down and is the single most-appreciated upgrade in a cold bathroom.
The transition at the door
Where the bathroom floor meets a different material, the transition is both a visual detail and a water detail.
A threshold that stops water crossing into a hallway floor of a less tolerant material is worth planning, particularly with a curbless shower where the floor is doing more work.
Large format on a bathroom floor
Large tiles on a small floor mean fewer grout lines, which looks calmer and gives less grip. They also demand a flatter substrate — any hollow under a large tile becomes a crack.
On a small bathroom floor with fixtures to cut around, large tile also produces a great deal of waste and a lot of cut edges. A medium format frequently looks better and installs more forgivingly.
Ordering quantity
Order 10 to 15% over the measured area for cuts and breakage, and more for a diagonal or herringbone layout where waste is higher.
Buy it all in one batch with the same lot number. Tile shade varies between production runs, and a box bought later to finish a floor is visibly different.
Keep the leftovers. A cracked tile in five years is repairable only if a matching tile exists, and it will not be in production.