what is the lowest seed to win a first round ncaa basketball tournament game?

Answer 1

and the answer to the actual question is the 15th seed is the lowest to win a first round game

Answer 2

Low seeds
Lowest seeds to reach each round since 1985 (Sweet Sixteen onward):
Sweet Sixteen: #14 seed:
Cleveland State in 1986
Chattanooga in 1997
Elite Eight: #12 seed:
Missouri in 2002
Final Four: #11 seed:
LSU in 1986
George Mason in 2006
Championship Game: #8 seed:
Villanova in 1985 - won championship
While lower seeds have made the Final Four in the 64-team era (as shown above), the University of Pennsylvania's 1979 appearance is notable as they made it as a #9 seed—out of 10 teams in their region. In fact, they defeated the #10 seed, St. John's University in the regional final, following three upsets by each team.
Additionally, in 1980, UCLA made the title game as an #8 seed (with 12 seeds in each region), a record tied by Villanova in the 64-team era. However, UCLA's run was later vacated by the NCAA

First-round games
No #16 seed has defeated a #1 seed since the field was expanded to 64 teams, though some have come close. Eleven #16 seeds have come within 10 points of a #1 seed, with five of them coming within 5 points. Two have come within one point. Only one #16-#1 game has gone to overtime (Murray State vs. Michigan State in 1990). The five #16 seeds that have come within 5 points of a #1 seed are:
Fairleigh Dickinson against Michigan in 1985 (4 points, 59-55)
Princeton against Georgetown in 1989 (1 point, 50-49)
East Tennessee St. against Oklahoma in 1989 (1 point, 72-71)
Murray State vs. Michigan State in 1990 (4 points, 75-71 in OT)
Western Carolina vs. Purdue in 1996 (2 points, 73-71)
Only four #15 seeds have ever defeated #2 seeds:
Richmond over Syracuse 73-69 in 1991
Santa Clara over Arizona 64-61 in 1993
Coppin State over South Carolina 78-65 in 1997
Hampton over Iowa State 58-57 in 2001
Since the inception of the 64-team tournament in 1985 each seed # has played a total of 88 first-round games.
The #1 seed has beaten the #16 seed all 88 times (100%).
The #2 seed has beaten the #15 seed 84 times (95%).
The #3 seed has beaten the #14 seed 73 times (83%).
The #4 team has beaten the #13 seed 70 times (80%).
The #5 seed has beaten the #12 seed 59 times (67%).
The #6 seed has beaten the #11 seed 61 times (69%).
The #7 team has beaten the #10 seed 53 times (60%).
The #8 team has beaten the #9 seed 41 times (47%).