Unlike TCW, Rebels has a linear narrative and can't easily be broken into arcs labelled "watch" or "don't watch." Nearly every episode adds something to the overall story; if not a big event with major ramifications, then backstory for a major character, notable development in character relationships, lore, etc. Some episodes that are closer to filler in terms of Rebels narrative have cameos from elsewhere in the franchise that make them worth watching.
Also keep in mind that because Rebels is ongoing, any 'essential episodes' list will probably be flawed because characters/planets/events from arguably skip-able episodes could play more important roles in future seasons.
I strongly recommend watching the entire first season and deciding from there whether or not to start skipping episodes. It's only 13 episodes (far fewer than any TCW essentials list I've seen) and, as seen below, most are recommended anyway.
All that said, if you're intent on watching the minimum number of episodes, the following is my list. The bolded episodes should cover the absolute basics of Ezra and Kanan's plots, as well as Ahsoka/Vader scenes. Other episodes are optional and will have a note explaining why they are included.
Spark of Rebellion
Droids in Distress*
Rise of the Old Masters
[Empire Day†
Gathering Forces]
Path of the Jedi
Idiot's Array*
[Call to Action*†
Rebel Resolve†
Fire Across the Galaxy]
The Siege of Lothal
[The Lost Commanders
Relics of the Old Republic†§]
Always Two There Are
Wings of the Master‖
The Future of the Force
Legacy
A Princess on Lothal*
The Protector of Concord Dawn§‡
Legends of the Lasat‡
Homecoming‡§
The Honorable Ones‡‖§
Shroud of Darkness
Twilight of the Apprentice
Bold Key episode
* Contains appearance of notable franchise character(s)
† Part of multi-episode arc (denoted with brackets) containing a key episode
‡ Contains important development/backstory for Rebels-only character(s)
§ TCW tie-in
‖ Features notable addition to franchise lore
[] Multi-episode arc