Specialized Support, the Ares Chapter

Ares' kit does not particularly allow for strong wave clear. It is suggested that Ares be paired up with other strong lane clear ADC's to make up for this. Ideal duo teammates would be Chiron, Medusa, Ullr, Neith, Rama, Hou Yi, or Anhur. Depending on opponents, I would suggest leveling your 3 first and then your 2 for early laning assistance. The extra defense will tend to keep your minions alive long enough to compensate for not having particularly powerful wave clear. Otherwise, just level your 2 first. Generally, smarter adc's will stay a little bit behind their tank so that you wouldn't be able to chain them, I would suggest landing a chain on their tank first to stack the first movement speed buff and then land the other two on their adc, or if you think you're in danger of missing them, walk past their tank and zone their adc away before applying the last few chains to their tank thus effectively cutting them off from their support. Getting ahead early will hamper the enemy's initiative especially if they intended to use a cc immune ult to escape your ultimate. Yeah, really, no escape.

While people playing Ares may find themselves caught up with the idea of creating as many aura items as possible to abuse Ares' passive, keep in mind THERE ARE NO HEALTH BASED AURA ITEMS which will lead to Ares becoming burstable if focused. Most of the time, a smart enemy team will run a 3:2 Phys to Mag ratio, two of which will tend to be very bursty in addition to the one mage with all around strong damage and some form of Crowd Control.

Gem of Isolation
It can be argued by some that since Ares' slow is already enough to be potent, it isn't particularly sensible for him to bump it by a measly amount. Allow me to reveal some of the math behind this, however. Each chain will slow by 15%, which by the third chain is 45%; an already incredible amount. Gem increases that to 70% which may as well be standing still.
45 vs 70%
Keep in mind your 3 and ult will also now have a 25% slow on them making it possible to chase even if you end up missing a chain. 250 health from this item is good enough to add to your tanking efforts, and allows you to build into a more potent defensive item afterwards.