I am a casual gamer and just want to relax when playing games, bear that in mind.
Deathsmiles has enough variety in the characters and a chosen path through the stages you play through including order of difficulty. This gives it good replay value, and it is the shmup I've completed the most. Not difficult to play through either if you don't want it to be.
Ikaruga has 5 levels (I think) and they are not very long if you don't care about score or difficulty. More of a job to get the most out of it and not very casual.
Don't bother with R-type, it's extremely unforgiving if you ever make a mistake.
Really enjoying Hard Corps:Uprising. Not through this yet, ignore the comments about difficulty, you can go through one stage at a time and upgrade things to make it casual. The variety in levels put a smile on my face, and the absurd situations. Joyous.
Consider Metal Slugs, they look great fun - though back-loggery means I don't have them.
So: Hard Corps:Uprising - demo is a fun play-through too.
Anyone have anything else to suggest ?