I usually start with a Doran's Blade then rush a Youmuu's - the armorpen improves everything he does and the active means that when you really need that extra oomph to chase down that 5 HP jailbait who's been stunning you all game or solo that Master Yi who thinks a 1.5 second stun isn't long enough for you to make him regret being born, you can yell "SURPRISE, I HATE YOU," hit 1, and wreck faces.
After that I upgrade my boots (which I usually get after Brutalizer but before Avarice Blade/finishing Youmuu's), usually to Merc's. Then I get Phage, the last damage item I buy for the entire game in most cases. From there out I then satisfy a Perverse Affection for activated items with a Randuin's and a Quicksilver Sash (though I usually skip QSS for FoN if they don't have an Ignite, or they have fewer ignites than characters who need igniting more than Renekton [*coughMUNDOcough*]). Also, if they don't have a lot of autoattackers I might go for a Double Heart instead of the Randuin's - 2 Hearts of Gold gives 400 HP and 54 armor as compared to Randuin's Omen giving 300 HP, 80 Armor, and 25 HP per 5, with the CDR and slowing effect as gravy.
I usually don't bother with an Atma's because Renekton's damage is horrendous even without it; he's the kind of champ who can build straight tank and still hit for 700 off a RageW with just a Youmuu's.