Actually, Necrons can be very good in battle, but only for experienced players. Generally they're really tough, although once a friend of mine won a game by destroying his opponent's Monolith in the first turn, with a single Lasrifle shot. He was lucky, but it still goes to show...Kukul said:Don't play Necrons, they are awfuly boring to paint and assemble.
Personally I use Tau. Space Marines are good for a beginner, since they're pretty much tough as anything, but when I got bored with them I moved onto Tau and found them much better, easier to use, and they suited my style more (I'm a shooty type of guy, rather than close up, and good with stealth which means my Kroot are a useful diversion if need be). What sort of style do you prefer, OP? If you like to wade in then Orks or Tyranids will suit you, if you prefer a shooty sort then go for Imperial Guard or Tau. Eldar are excellent for a stealthy type of playing style, and Necrons are good because they can often be ressurected in following turns which means you'll have a much better chance at overwhelming an opposing army. It's all down to your choice of playing style, at the end of the day.