Crisis Suits, or XV8's are the key point in the Tau army (imo). They have a wide variety of weaponry. It has 5 main weapon choices: flamer, plasma rifle, burst cannon, fusion blaster, and the missle pod.
Flamer: Effective at close range, and against low armored troops. Specifically Orcs, Tyranids, and Imperial Gaurd. Normally not fitted to a XV8.
Plasma Rifle: This is the most common weapon found on a XV8, as it is realtively powerful, good armor peircing, and has good range. Great against high armored troops, such as Space Marines.
Burst Cannon: This is a fast shooting raltively weak weapon. Effective against medium armor troops, with ok range. Effective against Eldar, Dark Eldar, and other Tau.
Fusion Blaster: An extremely effective close range weapon. Ver powerful, very limited range. Effective against tanks, skimmers, and other large vehicles/creatures.
Missle Pod: The 2nd most common XV8 option. Allows 3 long range, semi-power ful shots. Effective against Heavy/Medium armor troops.