Tub/shower valves can be bought with the rough-in valve and your choice of trim (but the big box stores often sell them together as a set). Even within brands, not all trim will fit all valves, so you'll need to remove what's there and start over. Assuming this is also used as a shower, codes require anti-scald technology in the valve. If it is a tub only, that's not required, but because most are used for a shower too, anti-scald is often in most of them.
There are two ways to get anti-scald (and some contain both): thermstatically controlled or pressure balanced. The objective is to keep the shower from both getting too hot (with a limit control) and to keep the mixture from getting too hot if someone say flushes a toilet, lowering the cold in the mix thus raising the temperature.
The pros here like Delta. They make both, so it's your choice. Personally, I like being able to set the temperature at your ideal, and never having to change it bacause most of them have a separate volume control. most pressure balanced valves turn on full force starting at cold and the further you turn it, the warmer it gets up to the limit stop. Your old one probably didn't have a limit stop.