Like everybody else says its all about what you want, like, feel inside etc etc. What do your friends/local group play? Try a contrast. I got into it with a friend and he went for dwarves so i took orcs. This was partly based on a couple of decades of role-playing and LOTR. We knew what to expect and went thusly forwards.
It all comes down to what you like, its really that simple. They all work well. I would maybe avoid the more specialist armies Lizards WOC and undead until you have a sound grounding in the rules.
Perhaps shouln't say this but most troops/unit types are basically very similar only the fluff and and couple of special rules seperate them eg not much difference between an empire swordsman and a chaos marauder stats wise