On Leads
Really Loves Gengar Leads: Lead Zam + Non-lead Jynx is fun.
Really Loves Jynx Leads: Lead Gengar
Loves Sleeper Leads / Starmie: Lead Jolteon
Hates Sleep Leads: Lead Jynx / Hypno

On the 6th Slot:
Loves Waters / Rocks: Mega Drain Exeggutor / Victreebel / Venusaur
Loves Rocks: Articuno, Kingler
Loves Rocks / Restlax: Dragonite
Loves Water and Ice mons: Run Zapdos / Non-Lead Jolteon
Loves Electrics: Try Golem / Rhydon
Loves Electrics and Rocks: Try Sandslash or Articuno
Hates Rocks: Double Electric, consider Dodrio
Never uses Starmie: Moltres

Miscellanious:
Loves Reflect Chansey, Alakazam: Slowbro, Leech Seed Exeggutor
Hates Gengar: Lapras, Cloyster, Jynx, Non-EQ Lax, Dragonite, Victreebel, and Pinsir are all worth considering.
Dislikes Exeggutor: Try Rhydon, Golem, Slowbro, Psychic + Thunderbolt Starmie
Loves Slow Teams: Lapras, also consider Nidoking and Tentacruel
Loves physical offense: Cloyster

Other things to look for:
Do they scout for Counter on Chansey and Snorlax often?
How early / late do they tend to explode?
How conservative are they with Chansey's HP? Having an idea of this is useful when trying to pressure Chansey with Snorlax and Alakazam.
How often do they engage in Chansey freeze wars?
Is their strength more in mind-games (such as with myself or Lutra) or in calculated (near-)perfect plays (such as with Marcoasd or Alexander)?
How conservative are they with thier Exeggutor? If they normally let it get paralyzed and take a couple of hits in the game especially when facing certain lead matchups, then using a team with Rhydon and paralysis support is a worthwhile approach to consider.