I may be alone in saying this... but if I had to nominate a game for "most underrated sequel ever", that prize would probably go to Ninja Gaiden 2.
Still a combat game at its core, NG2 corrects many of the flaws of its predecessor: gone are, for the most part, the long tedious platforming sections. Controls are mostly similar to the original, but feel tighter now, and enemies are ruthless and far more numerous. You'll be attacked from off-screen very often, but almost never to the point of unfairness. The big addition here is the Obliteration Technique, which allows you to immediately finish an enemy if one of their limbs has been cut off. While this is good for novices, power players will probably prefer to use the typical Ultimate Technique to make short work of their enemies. Just in case, you also have some powerful Ninpo spells. All of these attacks are not just for show: they give you some invincibility moments, which, if timed correctly, allow you to avoid some pretty damaging attacks. The result of all this is a particularly tight action game, where you're either blocking, charging UTs or cutting limbs and obliterating. The action is rarely interrupted by either the player or the enemies.
The health system has been revamped to feature a somewhat rechargeable health bar: however, it will only recharge when there are no enemies around, and not completely anyway. It's also balanced off by allowing you to bring far less healing items in comparison to the first game. Therefore, while battles would seem to be easier, for the most part they are not. Especially boss battles, where you won't really be able to herb-spam anymore, and your health won't recharge either. You can also choose to have fun with different weapons, though for the most part they are not important. But if you experiment, you'll find a lot to like. I'm partial to the Lunar Staff myself. It's good for making a mess.
With so much work gone into the combat system, everything else is mostly just decent. The story is ridiculous enough, and there are the usual buxom babes to oogle at - I wouldn't have it any other way. The graphics are ok, but it looks like an early 360 game, even though it came out in 2008. At least it moves an awful lot of enemies, and there's quite a bit of variety in the landscapes. The soundtrack is just ok, and so are the sound effects. But does all of this matter?
To previous players of Ninja Gaiden (or Black/Sigma), NG2 gives off a distinct feeling: that feeling as if Itagaki said "ok guys, we have to focus on the combat system, the rest doesn't matter". And so they did. In return for a passable technical presentation and weak story and characters, we get what is probably the best combat system of any game of its kind. The camera can still screw up at times, and the framerate doesn't always stay solid, but there really isn't anything better if you just want to cut some enemies and get your butt kicked in the process. The Acolyte difficulty is also pretty good for first-time players, because it's a bit easier than the Normal difficulty of the original game.
Cleared on Acolyte and Warrior difficulty levels. Tried on Mentor, couldn't even get past the first level. I'll leave that to the masters. Eh.