Nightmare is definitely NOT near the top character in the game, but they all seem to be pretty well balanced, except I'm pretty sure that Maxi and YunSung are proven stinkers. I don't think Raphael is that hot, either, but I'm open to being proven wrong...
Fun fact, arcade units of SC2 (and SC1 and all the Tekkens, I think) have a built in chart of most-played characters. At our arcade, Nightmare (13%-ish) is most popular, followed by Kilik (12%-ish), and then uh... I think the rest are sort of a close margin. Most people I see playing are Kilik, Raphael, Nightmare, and Ivy.
I would say, though, that Nightmare and Astaroth are by far the best choices against most players, because most players are "newbie idiots lol!11". If you get up by rapidly tapping an attack button, randomly throw out moves, or are just careless at all, a good Astaroth or Nightmare WILL bust you for 1/5 to 1/3 of your life with NO risk to themselves whaaaaaaat-so-ever.
So, in order to beat these guys, you need to learn these:
- Get up by tapping guard. This gets you up and blocking faster than a roll or attack or ANYTHING. This is most important, I think. Also, if you're taptaptapping an attack button, a good player will just smash you flat with some giant weapon.
- Guard a lot against these guys. There are some slow moves that you can punish.
- Against Nightmare, work on disrupting his "flow" - that's how (a good) Nightmare wins, by seamlessly and constantly moving from stance to stance with wacky combinations of highs and lows and sidesteps. You guess wrong and BAM! There goes a quarter of your life. Watch for obvious chances to guard impact, throw, or jab/kick quickly. Astaroth, however, works with lots of sudden starts and stops. Take advantage of any HESITATION he makes.
As a self-proclaimed Nightmare god (olol) the characters that give me the most trouble are Cassandra, Xiuanagngnhhwa, and Ivy. Xianghua is great for disrupting "flow" and has that obnoxious fake attack into throw. Ivy can control close and far distances better than Nightmare, so NM has to stick to 'mid' range and uh, hope he doesn't get violently cornholed by Ivy's giant combo that goes something like this: (jump+A+B, A, B, F+B). I might be a little off but damn, it hurts.