Goldberg's streak was huge ... he was bigger at that time than anyone in wrestling other than Austin... This was a golden era for way over, big money superstars, Hogan, Flair, Nash, Rock, HHH, Taker, Hart.... all of them were big draws and big deals in 98 but none of them was more over at the particular time than Goldberg, most of them were awfully close, but Goldberg's popularity natiowide was 2nd only to Austin at this point and "The Streak", although I feel it was completely mismanaged after he became Champ (why were his WCW Title Matches taking a back seat to Hogan's non title matches at PPV's in July, Aug, & Sept & sharing main event status with his non title match at the Halloween Havoc PPV in Oct, one of the company's signature shows ?? ) WCW did a great build heading into Starrcade, pitting him against one of the few stars who legitimately could have been believed by the audience as credible enough to beat him, and made good use of his own un-defeated streak in his career (Nash's year long run of victories as WWF Champ).
However, Sammartino losing after more than seven years as champ was unreal... there wasnt anyone who expected Ivan Kolloff to beat him though to be fair Kolloff had a great pedigree, the Kurt Angle of his day with his Olympic experience, etc. More over, doing cleanly in the middle of the ring was shocking, at least as much as when Bruno won the title in the first place vanquishing the legendary Buddy Rogers in under a minute.
I would say Bruno-Kolloff and Hogan ending Andre's undefeated streak at WM 3 were the biggest in the modern era of pro wrestling (Post 1970). Certainly from a money making stand point Hogan-Andre was the biggest, really helping to turn WrestleMania from nothing more than WWF's rip off of Starrcade into the veritable Super Bowl of Pro Wrestling, making it's significance as an annual event even bigger than many of the matches on it in years to come.
Goldberg-Nash was a distant third (At best)....
Some other memorable streaks worthy of mention...
Hogan not losing by pinfall for over 5 years (ended by Andre in the infamous Twin Ref Scenario with Ted DiBiase, setting up the WM 4 Title Tournament)
The Road Warrors/LOD not being pinned from roughly late 1985-1989 (ended by The Powers Of Pain I believe, LOD was asked to put them over before they left for WWF, if they were pinned during that time frame earlier by someone else please correct me, I dont believe they ever lost a pinfall during that time)
Ric Flair's Great American Bash 1986 Challenge... facing every top contender for the NWA Title, all over the country, different opponent in every city, in what was billed as the most ambitious schedule of title defensese ever by a World Champ... Flair won against Road Warrior's Hawk & Animal, The Rock & Roll Express' Ricky Morton & Robert Gibson, Ron Garvin, mega heel Nikita Kolloff, Magnum TA, Wahoo McDaniel, and Dusty Rhodes, all during the month of July, facing some of them on multiple occassions but running through every legite contender on the roster, wrestling as many as 5-6 defenses each week, only to lose on the next to last show in a Cage Match vs Rhodes, costing him his title in the process, ending his 2 1/2 year long run as NWA Champion
Aforementioned streak by Kevin Nash as WWE Champ - did he ever lose a match during that run, even by DQ, before dropping the title ?
Brett Hart's streak during his 1st World Title Reign - wasnt he undefeated that whole run, not even a DQ loss ??
Of course, maybe the greatest streak of all time, still active, Undertaker's 20-0 run at WrestleMania which has included al out anything goes bloody brawls with HHH & Ric Flair, World Title wins over Edge, Batista, & Sid Justice, a retirement match vs HBK... it's a remarkable run, maybe more so since WM 17 (some of his early Mania matches werent as significant, but he won them all, something Austin, Rock, HBK, HHH, Hogan, Flair, Foley, Hart, Savage, et all can not say). This will likely go down as the greatest streak ever, and if someone actually ends it (unlikely) it will top everything except maybe Bruno-Ivan & Andre's 10 year Undefeated Streak.