I am frequently asked for my opinion on the best surface for a tub or shower surround. There are a number of good choices that depend upon the look you are after as well as your budget. Here is my list and their approximate cost levels.
Natural Stone (granite, slate) or quartz (Silestone, Cambria) slabs. A hard solid smooth surface with tight epoxied and/or silicone joints is tough to beat. Slabs can be cut to a 2cm thickness to save money vs standard 3cm as for counter tops.
Solid Acrylic (Corian, HiMacs,) slabs. Again, a hard solid smooth surface with either 'hard joints' (those epoxied together) or lapping siliconed joints. Many new colors to choose from. For a shower the pan can be made of the same material.
Stone, porcelain or ceramic tiles. A traditional finish with an endless choice of colors, textures and finishes. Installation and wall prep is critical though. Make sure that walls are first covered with cement board, taped and mortared. If a steam shower is in use, an additional water proof layer is recommended before setting the tile with thin set mix.
Moderate to Expensive
Cultured Marble slabs (Onyx) is an option to not have grout lines and still get a smooth surface. Cultured marble slabs can be custom made for your tub/shower or purchased in kits. Though cultured marble is not a solid surface like quartz and acrylic it does hold up well in a tub/shower.
Fiberglass or other composite materials covered with an acrylic or polyester coat are clean, smooth, durable and cost effective. Many colors and styles are available. You will want to make sure and get a unit that is actually either 3-piece for the tub or 4-piece for the shower. One piece units may be too difficult to place in other than new construction. A good pre-formed unit is sturdy, self supporting and attached directly to the stud requiring no wall prep work.
Vinyl surround panels that must be glued to cement board and cut to fit. The most affordable alternative.
No matter which tub/shower surround you choose make sure and choose an experienced contractor that warranties their work. Most products work very well when installed according the manufacturer's specifications.