Why is Urza's saga the best.. well reason 1 is the one that invasion is a good runner up.
Really interesting story concept.
The secondreason lies in the cards. Urzas saga had such a MASSIVE amount of incredibly good cards that it now fetches the highest price for a booster next to the obvious overkill alpha/Beta, 15 $, and that's the average price for a booster of the Urza block, a Saga booster you can buy for an average 20$(or, dependingon shop you can buy 5-6 Arwhatever the new edition is for 1 Saga booster), even allicances doesn't get up to that range, it had some of the best lands and artifacts(or artifact related cards) around, many of wich where incorporating legendary concepts of the game, to the point where it is save to say that without urzas saga artifact decks would not be the same. Finally it was combo builders paradise.
But the real reason, and what it did the best was it gave all colors something really good.. or nasty. Blue got the fucking morphling(aka best creature for many years to come, and, in my opinion, only very rcently surpassed by the platin angel), red got the lackey aka goblin infestation,, artifacts got the academy wich turned out to be a terrible idea once mirrodin peeked around, Green got Geas Cradle(wich I still remember the 'contemporary gamers', back then complaining about day in day out, just as I was pointing at the Tabernacle I now point at it, but that's a different story), White got fucking 'oh hello, I'm 20/20, oh wait Lifelink? why thank you! I'm now 40/40' Serra's Avatar(and don't get me started on Worship in the right deck) and black... Only Yawgmoths will, aka 'I feel like playing my entire graveyard again trololol