Death Smiles is probably the easiest to get into, as you can choose difficulty and stages, but be warned: if you play all levels at the lowest rank the final level will look brutally hard in comparison. It's also quite unforgiving as far as scoring goes, because the skull counter carries over from one stage to another, so if you fuck up a recharge your whole run is ruined, kinda like Guwange. In games like Mushi or Ketsui you can make a mistake without affecting your score in the other levels.
I think Mushi Futari is another good game to start with, it has a ton of different modes and aside from the standard Novice modes (not really worth playing, imho) it has a very easy arrange mode and Original Black Label is quite easy to 1CC too (your shot is so powerful the bosses won't live long enough to be able to use their worst patterns). On the other hand, Maniac mode is damn hard and good luck if you try Ultra/God mode. Futari has everything!
