@InTheLandOfTheRisingSun monolith and hydorah come to mind - both gorgeously presented and quite tough
@InTheLandOfTheRisingSun oh wait i missed free. monolith isnt free uhhhhh
hold on one moment
@InTheLandOfTheRisingSun oh also linley henzell has some good ones https://www.allegro.cc/members/linley.henzell/projects
@InTheLandOfTheRisingSun and if you REALLY want a deep cut, there's A Damask Proposal, available on this page: http://www.umbrella-isle.com/games.php
it's legit one of my favorite shmups despite how obscure it is, just cos it's really well implemented and has good music