It's kinda weird how people seem to go on about how Black Mamba may have taken Nemesis' title...when really, Nemesis doesn't really have a title of being that well themed. Black Mamba beats nemesis hands down, but I believe Dueling Dragons (for example) is better themed than Nemesis.
Anyway, Mamba vs. Nemesis breakdown
Layout - Black Mamba has a better, more intense layout with a cool Jr Illmellman and (at least what I read) the tightest helixes b&m have ever made. Reports I've heard claim many found it too intense to ride again.
Station - Nemesis station probably looks better from the outside, but Mamba's interior is supposed to be fantastic. There are doors for when the train comes into the station. The inside is themed really well with cool wooden poles and fitting music as well as cool lighting effects. Nemesis has little interior theming with mainly just corregated steel.
Trains - Nemesis has no theming on the trains, Mamba has a cool pattern on the wheel covers and a web effect underneath
Theming - Mamba has loads of near misses. Apparently you can touch the waterfall on the Jr Ilmelmann, it comes tremendously close to the walls near the end. There is a bridge leading through the helix. Half of the lift hill and the pre-drop are enclosed in a giant building. The Jr Illmelman hugs a large rock with a face sculpture extremely tightly. They used African wood and have sculptures made in Africa for their theming (the Africans did rituals to bless the scenery before it was sent of to Phantasia land). Nemesis has some nice stuff such as the red water, paint effects on the track and its built into the landscape well.
Queue line - The queue line is beautifully themed with neat wall paintings, lakes with canoes and great views of the coaster. Nemesis has some ok views of the coaster, but little theming to keep you occupied in the queue.
Metro 8)