Ogre Kingdoms - This army has strong characters, good warmachines, are good at close combat and are fairly fast. Mournfangs and Ironblasters are considered underpriced. Their main weakness is having a fairly small number of expensive units in your army; if one gets wiped out by a big spell or some bad luck, you will be in trouble. They are not very heavily armoured and don't have particularly good leadership. Ogre Kingdoms have a very nice range of models and are quite popular in tournaments.
Warriors of Chaos - This army is possibly the strongest army in close combat and quite easy to play. The main risk is getting outmaneuvered by your opponent. With two hellcannons, two warshrines, big blocks of Marauders with great weapons and mark of Khorne, Warriors or Chosen with Halberds and Shields, magic items that screw around with the magic phase. there are some seriously nasty combos out there for Chaos generals to take advantage of. You can also theme your army based on different chaos gods, and have fun converting models to match.
Top TierThese armies have few weakness, lots of powerful or underpriced units, and may even put off opponents for being overpowered or 'cheesy'.