Oh goodness, there's so many to choose from...

Perhaps William Blake? His mythopoeic works are the first of that sort, and in that sense are an extremely early precursor to the sort of fantasy myth-making that Tolkien engages in. He also writes psychedelic, incomprehensible epics of which I am rather fond of even if they barely count as poetry: they do count as art though, all the typographic versions of his work miss the point that they were all meant to be PAINTINGS and ILLUSTRATIONS.

Novalis too, can count, for I am a lover of his Hymns to the Night, Hymnen an die Nacht, which I feel best express the sentiment of German Romanticism.

Of the British romantics proper, Lord Byron comes to mind because, quite simply, I really like epics and narrative poetry lmao and even if it's not always the smoothest read due to pseudo-archaisms, his Childe Harold's Pilgrimage retains some beautiful descriptions of European environments and I've often thought of what the equivalent of such an epic from other cultures might be like to read, cultures with different environments and poetic meters and standards...

John Keats too for his Pre-Raphaelite-like engagement with aesthetics, and Percy Shelley has many beautiful poems like Hymn to Apollo and Ozymandias, as well as his long works... that said, as my mind has congealed with time and lost the ability to write poetry, I find myself going back more to either prose writers like the Brontes, or the Gothics if you count them, like Poe, because they meet my moody disposition best.