Truth be told, I struggled with coming up with candidates for this category...so I'm really hoping you'll help me out here with candidates for best in the secondary. Truth be told, I think Nebraska has had more talent in the secondary in the last fifteen years than in the previous hundred...but that could be an misperception on my part.
So I've added several of your suggestions to the bottom of the list...though I'm sure I could have added even more to this list...but I've got to draw the line somewhere. So Daniel Bullocks, Tyrone Williams, Toby Wright, Keyou Craver, Brian Washington, Fabian Washington, and Bruce Pickens...sorry, but I had to draw the line somewhere.
So here are my six candidates; if you still disagree, go ahead and vote for the "somebody else" option.
Mike Brown
This one was the no-brainer selection...especially when you consider his NFL career. He started 38 games for Nebraska from 1997 through 1999, and was a first team all-American his senior season. His name still sits atop the career charts for defensive backs: tops in total tackles in his career (287), unassisted tackles (137), assisted tackles (150), tackles for loss (17). He's had a heck of an NFL career despite annual season-ending injuries from 2004 through 2008.
Ralph Brown
Like his teammate Mike, 'ol Ralphie was an all-American in his senior season at Nebraska. He started every year at Nebraska, and is tops at Nebraska in pass breakups in his career (50) and fourth in interceptions (11). While he hasn't had the notoriety of Mike Brown in his NFL career, he's entering his twelfth season in the league. He could also deliver the mean hit; just ask Jamaal Lewis..
Mike Minter
His Husker career was hampered by a knee injury in 1994, but his 10 year NFL career spoke volumes. An all-conference safety his senior season, he moved up to linebacker to replace Terrell Farley, who'd been dismissed from the team. Didn't find any highlights from his Husker career...but how about this from his days with the Carolina Panthers?
His 84 yard interception return in 1996 against Michigan State is ninth longest in Husker history, and his five interceptions in '96 tied him for eighth place in Husker history for interceptions in a season. He was fifth on the team that season with 51 tackles.
Barron Miles
The Red Barron wasn't the biggest guy on the field, but Barron Miles played that way at times. He was all-Big 8 his junior and senior seasons, and was a third-team all-American. But Miles made his name on special teams, where he's still the Huskers all-time leading kick blocker with seven. (Big Mister Suh only got 6 in his career...) I have two favorite Miles moments. First, in 1993, he caught the ball off the foot of the Oklahoma State punter in the end zone for a touchdown. Officially, it was a blocked kick...but really, he didn't block it...he caught the damn thing. The ball never left his hands. It was the "Play of the Week" on both CNN and ESPN that week. The second one was most touching when on senior day, after he was introduced, he knelt down and kissed the turf in Memorial Stadium to show his affection for Nebraska. The only time I've ever seen that.
Miles was a 6th round NFL draft pick, but he made his career in the CFL,, retiring this spring. He's the CFL's all-time leading kick blocker (13) and second in career interceptions.
Josh Bullocks
Josh Bullocks was absolutely sick working with Bo Pelini and Marvin Sanders in 2003, intercepting a school-record 10 passes that season. His 13 career interceptions is second all-time in Nebraska history. He was an all-American as a sophomore, though only honorable mention in 2004 after Kevin Cosgrove replaced Pelini. Not willing to subject himself to that for another season, he declared for the NFL draft where he was a 2nd round (40th) pick of the New Orleans Saints. Coincedentally, his twin brother Daniel was drafted the following season by the Detroit Lions in the second round with the 40th pick. After four seasons with the Saints, Bullocks spent last season with the Chicago Bears.
Bret Clark
Bret Clark was a first-team all-American in 1984, and a honorable mention all-American in 1983. He's third all-time in career interceptions with 12, and also added five fumble recoveries in his career (tied for first all-time). He's also tied for eighth in career pass breakups. He also set the record at the time for career unassisted tackles by a defensive back. He was drafted by the LA Raiders in the 7th round of the draft, but played three seasons for the Atlanta Falcons.