The actor spoke at a panel for the Edinburgh International Film Festival in Scotland Saturday ... and, he took a similar line to many Hollywood legends -- saying cinema's gone downhill in recent years, and Marvel's at least partly to blame.

The way Cox puts it, the grandiose nature of the movies has led to a loss of plot ... and, actors are treating it more like one big party than a real challenge.

He specifically name-drops Hugh and Ryan here ... saying "Deadpool & Wolverine" might make the studio a ton of money, but it doesn't really move the needle creatively.

Cox says he thinks "Hugh Jackman can do a bit more" and so can Ryan Reynolds ... but, they choose not to because they're getting paid a ton.

No knock on them for that, BC adds ... so, he's not saying Hugh and Ryan are doing anything necessarily wrong -- just that they could be challenging themselves more.

Cox recognizes it's weird he's mentioning this since he's done the whole superhero thing before ... appearing alongside Jackman in "X2" way back in the day -- but, adds even he forgets about that role pretty often.

Of course, Brian isn't afraid to speak his mind ... coming under fire multiple times this year for blasting fellow actor Joaquin Phoenix and slamming organized religion too -- basically, Cox isn't one to shy away from controversy.

Worth noting ... 'D&W' has grossed over $1 billion worldwide already -- so, Hugh and Ryan aren't likely to listen to anything Cox has to say.