Have 2004 ford f-150 FX4 with a 2" leveling kit and factory 18" rims. What is the best size tire to put on?

 

best in regards to what?

id go 275/70/18 on stock wheels. thats roughly a 33 inch tire and wont be too wide for the wheel. ive had 33x12.5x18s on stock wheels and it just looks funny imo, plus it can cause multiple problems. as far as brands, toyo and nitto are great choices for all terrains and mud terrains

 

you can fit 35x12.50 18 but on stock wheels, you will need a set of wheel spacers or they will rub on the upper control arm slightly if turned too sharp.