Somebody please tell me I'm not the only one who typed 'denki mirko fanfiction' at three in the morning and stared at a wall of questionable tags. Look, the pairing's got a weird energy that doesn't always get served on the big platforms. AO3 is your obvious first stop, the tagging system is unparalleled. You can filter by 'Kaminari Denki/Toga Himiko Mirko' and then sort by kudos. That'll get you the crowd-pleasers, the popular tropes like 'pro hero mirko x lightning dumbass' stuff.
But the real interesting bits? Sometimes live over on dedicated 'My Hero Academia' Discord servers. Smaller communities where writers feel safe to experiment with power dynamics that are less... conventional. I've seen a couple where Mirko's mentoring isn't just physical, it's about channeling Denki's overcharge in ways that get emotionally intimate, which AO3 summaries often flatten into pure smut. Tumblr's a mess for searching now, but if you find one writer you like, their blog might link to a Carrd with all their works. It's more of a scavenger hunt, honestly, but the quality can feel more personal than the sometimes-formulaic multi-chapter fics on FFN.
Honestly, 'best' is subjective. If you want polished, lengthy romance with all the beats, stick to AO3 filters for word count and completed works. If you're after a rougher, more chaotic character-driven moment, you gotta dig deeper into fandom spaces most search engines never see.