I would say that all season 1 is good; it’s only 14 episodes (special overture + 13 regulars) but it’s worth watching every one of them. But season 2… yep, too many fillers… so, I would advise you to only watch the following (12 episodes):
(2.01) - The Siege of Lothal
(2.02) - The Lost Commanders
(2.03) - Relics of the Old Republic
(2.04) - Always Two There Are
(2.05) - Brothers of the Broken Horn
(2.12) - The Protector of Concord Dawn
(2.15) - Homecoming
(2.16) - The Honorable Ones
(2.17) - Shroud of Darkness
(2.18) - The Forgotten Droid
(2.20) - Twilight of the Apprentice, Part 1
(2.21) - Twilight of the Apprentice, Part 2
It really should have stayed a 14 episodes format, with the good material of the 9 bad episodes infused into the others, the good episodes would have been even better. So instead of 14 very good episodes it’s just - for me - 12 good episodes (with a handful of great ones) and 9 mediocre (at best) ones. Still a better show than the overrated TCW.
edit: it seems I mostly agree with NeverarGreat here. Except for Legend of the Lasat (logicial since I don’t like ST5 😃).