Finding a steady spot for Marvel imagines can be a whole quest in itself, depending on what flavor you're craving. Tumblr is probably the original powerhouse for that short, punchy, reader-insert style; the tagging system means you can dive straight into your favorite character or trope, though quality varies wildly. It's got this chaotic, living archive vibe where writers build little communities in the replies.
I've also spent way too much time scrolling through Archive of Our Own (AO3) using the 'Reader' and 'Marvel Cinematic Universe' tags. The filter system there is unmatched—you can sort by kudos, word count, even exclude certain tropes. The writing tends to be more polished and narrative-driven compared to Tumblr's snippet style, which is great if you want something with more meat. Wattpad is another option, especially for mobile reading, but you have to wade through a lot of... let's call them less refined works, to find the real standouts.
Honestly, my reading shifts between all three. Tumblr for quick hits and mood boards that set the scene, AO3 for when I want a proper story, and Wattpad on my phone when I'm just killing time. The 'best' one really hinges on whether you're after a five-minute daydream or a full-blown alternate universe epic.