The Frieza saga easily produced the best fights of the series. It's where it should have ended. Frieza was the ultimate bad guy. He was the ruler of the universe, stronger than everyone and pure evil. Plus he was actually a part of the story, not just some extra strong bad guy who pops out of nowhere to fight Goku and the gang.
The reason it was so good was because Frieza really was the bad guy, the show presented him as the final obstacle. He had enslaved the saiyan race (as well as whole other galaxies), and you really did despise him for that. Everyone had a grudge against him, even Vegeta turned on him in hopes of avenging his race. All the fights leading up to Goku's confrontal with Frieza were the best in the series, everyone had a chance to shine, and you really rooted for the good guys to overcome Frieza.
And of course, there is the final fight, Goku vs. Frieza. When Krillin died, and Goku transformed, that was the beginning of arguably the best fight in the series. There was no ridiculous transformations, and people weren't being wished back every episode. Krillin was dead, Goku's best friend was gone for good. And this pushed Goku over the edge, putting him in the legendary state, he became a SSJ, and the ultimate fight began.
It ended perfectly too, Goku defeated Frieza, but unfortunately Frieza's final blow to the planet was too much for Goku, and he ends up dying in the blast (At least, that's what was supposed to happen. But unforunately the series continued, and some bull**** story was made up to give Goku an excuse to live).
It's hard though to announce that single fight as the best in the series, I myself enjoyed every fight with Frieza equally. Vegeta's first fight with Frieza. He actually matches his power, which completely surprises everyone. Piccolo's fight, where his spectacular power somehow brought him head to head with Frieza in a dramatic clash. Vegeta's second battle with Frieza. He puts up a last ditch effort to defeat him in his most powerful form. Despite the overwhelming odds, he still tries his hardest, which ultimately ends in his death.
Too summarize, i'm gonna have to say the "Frieza vs Everybody" fight was the greatest. It really was one big fight, despite being somewhat broken up. If any of the fighters were missing, then Frieza would have won and ruled the galaxy forever.