My personal favourites are/would be:
Team Snipers (S7s Only)
Rumble Pit - Snipers (S7s only), Escalation Slayer (Gun Game), One in the Chamber (Tactical Slayer, spawn/respawn with one bullet and kills generate a bullet), Fiesta, Ninja Slayer, Rock N Repulsors.
Big Team Social - Fiesta Slayer, Team Snipers (S7s Only), Snipers and Shottys, Skockets.
Team BRs - Slayer, CTF, Strongholds, Land Grab, KOTH, Oddball, VIP, etc.
Action Sack - Ninja Slayer, Kong Slayer, Mini Slayer, Low Gravity, etc.
Arena - 4v4 Slayer, Strongholds, Oddball, CTF
Lone Wolves - 6 Player FFA Slayer
Doubles - 2v2 Slayer, Oddball, CTF.
Rotating Playlist - 4v4 Survivors, 2v2 Dodgeball, 4v4 Snipers (S7s Only), 6/8 Player FFA Snipers (S7s Only), etc.
I think that Halo has always offered a great level of variety and options for both social and ranked matchmaking (on top of having the custom games browser too) so I hope to see this in Infinite too.