Most mountain bikes have Shimano shifters & derailleurs. SRAM is the other main vendor. Shimano makes a wide range. Below are some models of Shimano rear derailleurs.
SHIMANO TOURNEY - Mainly designed for riding around the neighborhood.
SHIMANO ALTUS - Sufficient for paved or very easy trails; avoid hills.
SHIMANO ACERA - Slightly better, but won't shift well under stress; capable for easy casual riding.
SHIMANO ALIVIO - Minimal capable for recreational riding; will handle some hard riding.
SHIMANO STX / DEORE - Entry-level components for hard riding; tough & reliable.
SHIMANO LX - The minimum standard for hard riding. Reliable & responsive.SHIMANO XT - More responsive than LX, but still reasonable tough.
SHIMANO XTR - Light, responsive racing components; tend to be quite expensive; often sacrifices toughness for responsiveness.Look for a component set that fits your expected riding style.
