Actual Good Filler

The Asgard arc in Saint Seiya is pretty damn awesome. The fights got more dynamic and it took a cool spin on things, it fits in so naturally it was crazy too. I actually think the Garlic Jr arc in DBZ is pretty fun... we get to see more badass teen Gohan and Piccolo, topped off with the cool Kami Tower setting. It's not incredible and maybe something I skip with rewatches, but compared to most filler out there... it's good to me.

One Piece has good filler, if you can call it that. Naruto Shippuden has okay filler, but it's nothing great, especially all the flashbacks later in the series.

Full Metal Panic's "The Wind Blows At Home" story arc was basically filler because it wasn't in the light novels. It's a great story arc that fills out a lot of Sousuke Sagara's back story and is incredibly heart rending as you see Sousuke's homecoming to Helmajistan.