I like the more ridiculous AUs... like, the more far-fetched and fantasy like the plot, the happier I am. Basically, I want the characters to resemble their namesakes as little as possible.
Superheroes Suck, or, The Unfortunately Extraordinary Life of Jensen Ackles by cleflink was right up my alley. Superhero-AU where Jensen doesn't end taking after his famous parents and instead opens a nice florist shop and talks to plants... it's adorable, ridiculous, and since there's superpowers in it, it's pretty goddamn awesome too.
Unlike with Supernatural, I like my Sherlock slashy - specifically Johnlock. That being said, only one of these is slash...
the sensation of falling as you hit sleep by greywash - I can't for the life of me remember this fic, but I liked it enough to bookmark. It appears to be a post-Reichenbach fic that builds to an eventual reunion and Johnlock. It looks like I'm not the only one who liked it! Also, hey, it's part of a series, and I don't think I've read the rest of the series. Anyway... um, check it out, I guess. Here's someone else describing it.
An Avalanche of Detour Signs by gyzym - A het fic! This is a Molly-centric fic that is excellent and awesome. Molly is a BAMF without losing the fact that she's not really a BAMF, if that makes any sense. It really does justice to the character, I thought. The only warning is that there's a bit of sexual trauma in the fic (due to exploring Molly's brief relationship with Moriarty) - I thought it was EXTREMELY well done, but if anyone has actually experienced a rather traumatic sexual experience they might have trouble reading that part...(it's not non-con, but it's not nice either...kind of really uncomfortable sex that you horribly regret afterward).
Seeds by thesardine - this is an really nice slow Sherlock fic. It's labeled as "gen-ish"...which means a lot of love, but no actual romantic relationship. The fic follows Sherlock through a "black mood" and how he slowly slowly rises out of it (or at least begins to) with the help of an accidental botany project. It made me smile. I mean, how can you not smile at a fic that starts off: "Everything was so heinously, viciously, malignantly, abhorrently, loathsomely, pestiferously dull. Despicably, deplorably ,deleteriously dull. Monstrously mundane. John was puttering away at his pestilential place of employment while Sherlock languished…loquaciously. Yes."
Fanmix - J.W. by devinleighbee (there's a link to their tumblr where you can download it in the sidebar) - This fanmix is fuckin' great. I absolutely love it. I don't usually download fanmixes, but since devinleighbee had a youtube playlist, I decided to give it a listen without committing to a download, and I'm really glad I did. The only change I made was to take out the Jackson Five song, because I personally don't like it. Other than that, it gets a little bit Mumford and Sons heavy by the 3rd act, but I mean... it's Mumford and Sons, I'm not really complaining.
Just like Sherlock, I like my Merlin to be slashy - this time, with Merthur (Merlin/Arthur - because I do like to go for the most obvious pairing. I mean, goddamn, they're actually canon if you translate all the metaphors.)
The Dream of Merlin Wledic by anon - this is a Merlin kink meme fill, so you have to read it across 47 comments, but it's worth it (or at least I think so.) It follows Merlin as he basically descends into madness, forced to dream about killing Arthur every night and unable to tell dreams from reality. It's a VERY engrossing story. It does leave subplots unresolved - but so does every episode of Merlin, so there's nothing new there! The ending might also be less dramatic than you expect, but I loved it.
No Law by deepsorskies - This fic is WIP Arthur/Merlin/Gwen fic but it can actually stand the way it is now as a finished fic (though it'd be even better if it continued.)
The nice thing about Modern AUs in Merlin is that you don't actually have to watch Merlin to read them. Because the author has to set up the modern universe, they'll reiterate who everyone is in relation to each other. So I think you can enjoy them whether you're in the fandom or not.
A Binding Contract by eldee_fic - this one is 99k and about sex-trade. Yay! It's for those of you who secretly, or not so secretly, like slave-fic or urban fantasy...or hopefully both. In this universe, magic-users are oppressed and fitted with bracelets that restrict/block their power. The fic follows Merlin, who becomes a sex worker on a short-term contract in order to... pay rent? I kind of forget that part. Needless to say, he ends up belonging to Arthur. and as a result Arthur starts to see how crappy society actually is.
Natural Selection by minor_hue - this is just a cute meet-cute story where Arthur goes on several disastrous blind dates at the same restaurant and ends up falling for the waiter (who is, of course, Merlin.)
A blind eye sees only memory by deadfics - This is actually RPS AU (Bradley/Colin aka Arthur/Merlin with different names, because an AU is an AU). Anyway, it's the fanfic classic amnesia fic, where one member of a couple gets amnesia and the other member moves into the guest room and pretends they're just friends for some reason... the thing I liked about it is that it's told solely from the amnesiacs perspective, and at one point you realize that he knows full well that they're a couple and his actions aren't necessarily what you think they are...
Dear Arthur by snowblood7 is another fanfic trope, or rather the Cyrano de Bergerac trope. Arthur moves away and Merlin writes him emails and letters while pretending to be Morgana, the girl that Arthur had a crush on. Arthur eventually comes back for a visit, eager to see Morgana, who he's felt so connected to over the years through her letters... The thing I loved about this fic is that it's not clean. It's messy and kind of horrible, basically, it's realistic for how people would feel in that situation (or more realistic than most fics with the same premise.)
Of Essays and Destiny by anon (on DW) - this fic follows Student!Arthur and EnglishTutor!Merlin as Arthur tries to write essays. To tell you the truth, I don't remember that much specific about this story, but I bookmarked it - so I must have liked it a great deal. I do know that it has great literary quotes throughout it and it's kind of beautiful that way.
If you want EVEN MORE recs, I've mostly been reading fic on AO3 these days, and I bookmark things I like with their in-site bookmark feature. I tend to put little notes to myself to remind me what I liked about the story too. So, they're like little not-very-well written reviews.