Take a bruiser who scales really well with CDR and build a Black cleaver as your second item.
The 20% CDR is really strong early as most of your spells (definitely your ultimate) have long cooldowns and it will make a big difference. The Stacking armor reduction will make your adc (and yourself) do a lot more damage once teamfights start. The sticking power of phage will let you chase and apply stacks.
Especially when you're facing a 3 tank team, black cleaver is a must buy. With the 20% CDR it's feasible on a lot more champions than before now. Add a BotRK for attack speed to apply the stacks and % health damage if they stack health instead of armor and you're set.