I’ve been asked, like, twice (literally). So it counts as a FAQ, for me!
The absolute starting point is still, always, and forever At Least There’s the Football, by Sheffie Sharpe. It will take you a while to get caught up. Love every damned minute of it. It’s what convinced me of Mystrade.
If you’re testing the waters, not sure if this is your One True Pairing yet, you might like to try elfbert’s Impact. It also scores for paying homage to Different for Girls, one of Rupert Graves’s best roles, I think.
Now let’s hit some Macpye ones. You all know Macpye illustrates the hell out of The Boys. This includes askgreglestrade. There are some short comics and things by Macpye all through his blog, but I would single out these: Dressing Casually, Drunk Dialing (I’m linking to the audio post for that one, even, because… no, really, if you want to hire Macpye to record things for you, I won’t be surprised. VERY GOOD VOICE), and Mycroft’s Handy Fangirls.
Now I’m just going to list all of mine since I’m a greedy, selfish, evil bastard.