Ninja Gaiden: Black is hands-down the best in the franchise, and it's a lot better than Sigma. It is also the closest to Dark Souls in the series, though obviously there are many differences. The challenge is fair, but it will require you to get good or get dead. You won't have many (if any) "Wow, I got lucky and made it through that part," moments. It's a mixture of action and adventure and requires good reaction times and well as smart tactics and preparation.
If you're looking to get enticed with fast/flashy action and grandiose environments, NG2 for 360 is your best bet after that. The gore is very beautiful and gives a sense of unparallelled carnage along with your accomplishments. Combat is not as refined, technical and balanced as it is in Black, but it's still good and it is much faster. NG Sigma 2 has some nice features added in (new characters, level select and the such) but the gore and difficulty is toned down.
If you're looking for features, Ninja Gaiden 3: Razor's Edge has probably got the most. It has plenty of playable characters, costumes, mission modes, level select, co-op/multiplayer modes, etc. It's also pretty fast-paced and gory. However, this comes at the sacrifice of gameplay integrity. A lot of it is cheap/random/mashy, much more so than NG2 or especially Black. NG3 (vanilla/original) is horrible. Don't give it a second glance.
Let us know what you choose and how it goes!
edit: If you're interested in a full breakdown of the pros and cons of each game (at least in my own personal opinion) you can see
here
