Tyranids are good in numbers and close combat then the weapons that they can use for long ranges combat.
Space marine are good at long ranges, middle ranges and close ranges if you have got the right units on the battlefield at the right time, and you have some Imperial knights in you army.
Imperial knights is one of the armies that you can't defeat by a small army or if you don't have big weapons because Imperial knights are hard to defeat and only three of them can defeat a small army.
Tau empire have some big weapons that can do damage but once the units that have those big weapons have gone you have a chance of wining but it is very slim. If you are playing with a opponent that has an army of Imperial Knights then you need all the big guns that you have got.
Necron are the army that have more flying units than any other army and they only have walking unit and flying units.
Chaos space marine is like Space marine but more evil and have units that space marine have not got or the units are a bit ugly.
Orks are a army that know little of close combat or shooting from a long or median range, they can't think of how many Orks that they will need so they just send as many Orks as possible to attack.
For More armies go to the game workshop store or website and see what units you can get of your army.