In a game like this, there is no "best" unit, it all depends on how much they cost you and how well you use them.
Warbands are a decent basic spear unit, great for stopping cavalry, chariots and elephants.
Woad Raiders have good attack but poor defence stats. They are cheap and good for flanking, but they do generally take quite high losses.
Swordsmen are a basic melee infantry unit with good morale, good attack and relatively low defence.
Slingers are a useful missile unit, great for taking out lightly armoured enemy units. Don't place them behind your lines though.
Head Hurlers have a huge AP ranged attack, so they are great for taking out Roman heavy infantry.
All three types of chariot fair very well against cavalry, but relatively poorly against infantry compared with other cavalry equivalents. Keep them moving and use them to scare infantry and you will do well. Allow them to stop close to an enemy formation and they will be butchered.
Druids are great for keeping morale up. They scare the enemy and give your men heart. They are also good fighters, making them an excellent choice as reserve troops behind your battle line.
Chosen Swordsmen are your elite infantry. They are basically Swordsmen but better in all respects.
Warhounds are attack dogs, fun to use and great for scaring the enemy. The best use for them is drawing enemy generals out of wooden-walled towns. Surround the gates with spearmen, send in the dogs and when they attack the enemy general, he comes rushing out at the handlers who you have placed behind the spearmen. If you get this right, you should be able to slaughter a faction leader's entire bodyguard in five seconds. It works with Hastati as the Romans too, although not quite as well, since they don't have an attack bonus.