i liked the first onimusha more than the third. i can remember thinking the atmosphere was better, and back then it was more impressive. not sure if it held up well tho.
the second onimusha was my favorite. everything was better than on the other ones.
the third was pretty mediocre. less characters than the second, the character switching thing was lame, that fairy thing or whatever was annoying and it was too easy. this one required too many orbs to max out all the weapons too iirc, so i had to grind a little in the evil palace or whatever it was called.
i only played the fourth one briefly at a friend's house, but i didn't like it. it was like a poor man's devil may cry instead of a better resident evil.
as for other suggestions, i'd say zoe2 and kingdom hearts. kh is more action than rpg. i like it, but if you can't get past the disney characters, it isn't for you.