This episode is largely agreed to be the best jump-on point.
http://pocketmonsters.edwardk.info/%5BPM%5DPocket_Monsters_Best_Wishes_Season_2_Decolora_Adventure_20_My_Dream_-_Pokemon_Master!%5BH264_720P%5D%5B25F857B9%5D.mkv.torrent

It sets up the XY arc and XY episode 1 assumes you've seen it, so it's a better place to start than that.

Also, here's the viewing order pasta from /vp/

http://pocketmonsters.edwardk.info/#PocketMonstersXYSubbed

Airing/viewing order:
Best Wishes Decolora Adventure 20 (not XY, but it's recommended you watch this first, since XY1 has a lot of callbacks to dialogue from it)
XY0
XY1-11
Complete Overview! Pocket Monsters XY Special!!
XY12-21
Best Wishes Season 2 Decolora Adventure Special - Iris VS Ibuki! The Road to Dragon Master!!
The Strongest Mega Evolution Act I
XY22-35
Movie 17 Guide
Movie 17 Premiere Special with TV Special
XY Movie 1 Short
XY Movie 1
XY36-49
The Strongest Mega Evolution Act II
XY50-54
New Year's Eve 2014 Super Mega Special
XY55-64
The Strongest Mega Evolution Act III
XY65-78
Movie 18 OVA
XY79-81
Movie 18 Appear! Special
XY Movie 2 Short
XY Movie 2
XY82-93
The Strongest Mega Evolution Act IV
XY&Z1-9
XY&Z Complete Overview Special
XY&Z10-33
Movie 19 Guide
Movie 19 Premiere Commemoration Special
XY Movie 3
XY&Z34-37

The six Hoopa's Appear! Operations specials aired alongside XY69-74

Note that the Strongest Mega Evolution specials are crucial to the plot, not optional side stories