Definitely Story plot and art 😃 ...like it even more if it is Ecchi genre 😎

Simple list, but these few and far between series I've come across:

• engaging story and plot (cliche or otherwise)
• characters that actually make me give a damn about what happens to them (i.e. I'd actually want the good guys to win and/or the bad guys to lose.)

if applicable then also:

• humor that makes me lol in real life or at least leaves me with a smile on my face
• tragic events that would bring me on the verge of tears or yelling "NOOOOO!"

As for art, I've come across awesome series with what I would otherwise consider good art and horrible series with good art, so I can never say that the art makes a manga.

"When we remember we are all mad, the mysteries disappear and life stands explained."

• Samuel Clemens/Mark Twain

An addicting storyline that involves magic.
Comedy like 1/2 Prince.
A heroine that can actually fight.
A plot that is totally unpredictable.