Pick the one you like the look of. Because, you will end up painting them if you want to game, and at the end of the day, it doesn't matter if your space marines kick ass on the table. If you hate painting them, then you're in for a very long haul...
If you absolutely must have a recomendation, then go with necrons. They're pretty much idiot proof. You point your shiny skeleton soldiers in the general direction of the enemy, and pull the trigger. And then drop a monolith in the middle of the board, and cause pandemonium on a global scale. There, enemy annihalated. *shakes head sarcastically* A good newbie army anyway...