For raw OPA, undead takes the cake due to having a trainable 9-point elite and not needing farms, thus freeing land for buildings you actually want. They also have immunity to ambush, plague immunity and spreading, and a/m potential. The weakness I see is not just the science penalty, but that combined with the cost of training elites diverting even more money from science.
Orcs are easier to set up economically than undeads and have a similar but cheaper army, but -10 BE is a harsh penalty... probably harder to deal with than -20% science. Still good at what they do and a decent choice for heavy attackers that want to use science and have power.
Avians are going to be the all-around best choice for pure attacking due to their attack time and lack of any strong economic penalty, even though their army is only 7/6. Their gains penalty only means that they're about even with orcs for damage inflicted, not accounting for the offense of orcs allowing for more double-taps or hitting of highly defended provinces. Of course it's also possible to forgo barracks, such that avian has the same attack times as other provinces but with more other stuff to make up for it. It can be played with in more ways than just attacking faster...
Dwarves are just as they've been - good at getting and staying big, with easy economic bonuses that work even if your peasants get fireballed. They have more utility as cows and economic provinces, but can work in war.
Humans are similiar to what they are now, not the strongest attacker but with lots of money and a strong enough army to stand up to the orcs and undead. Their penalties don't matter for much for an attacker, as -20% magic effectiveness won't change the ability of a/ms or t/ms to fireball you to nothing, nor does it matter when casting on a heavily massacred target. I've known humans to pull of some maging despite the penalty against non-massacred targets, though it's necessarily limited to the easy ops. The only heavy attacking race that can make a/t work very effectively.
Then there are the Elves that can only fill a very specialized role in the kingdom and kinda suck when all is said and done...
Halfers' weak armies make them poor attackers no matter how they're played, though they're not as bad as some people think due to the extra population. Alot of people are going to think, "durrr, I should use ospecs and keep a static elite defense because it's more nw efficient!"... this is going to be their downfall, and more likely the halfer is going to have to use their elites on offense, especially when they can hit provinces with low defenses. If they're played smart they can do alright... they're pretty much obligated to play a/t with strong TPA. Certainly not a race I'd want as the majority of my kingdom, but having a few halfer/rogues or halfer/warriors is still viable.