In 1991, LEGO released 11 Basic sets across 3 subthemes, all of these sets have been retired. Subthemes for that year included Seasonal, Supplementaries, and Universal Building Set. For that year, the total price of all retail sets cost $104, today those sets are valued around $558, a growth of 435%.
Most valuable 1991 LEGO Basic set
1666 Brick Vac $159.61
Largest 1991 LEGO Basic set
1882 Large Play Bucket 449 pieces
Most owned 1991 Basic set
1978 Santa Claus
11 sets

819 Blue Baseplate

Theme / Subtheme Basic / Supplementaries
Year 1991
Pieces 1
Availability Retired
819 LEGO Blue Baseplate
Retail $5.49
Value $17.67
Growth  +221.9%
Annual growth 3.4%

1474 Basic Building Set with Gift Item

Theme / Subtheme Basic / Universal Building Set
Year 1991
Pieces / Minifigs 69 / 1
Availability Retired
1474 LEGO Basic Building Set with Gift Item
Retail $9.99
Value $51.76
Growth  +418.1%
Annual growth 4.8%

1666 Brick Vac

Theme Basic
Year 1991
Pieces / Minifigs 132 / 1
Availability Retired
1666 LEGO Brick Vac
Retail $9.95
Value $159.61
Growth  +1504.1%
Annual growth 8.3%

1678 Building Set Special Offer

Theme Basic
Year 1991
Pieces 44
Availability Retired
1678 LEGO Building Set Special Offer
Retail $7.99
Value $8.88
Growth  +11.1%
Annual growth 7.3%

1698 Basic Building Set Special Offer

Theme Basic
Year 1991
Pieces 24
Availability Retired
1698 LEGO Basic Building Set Special Offer
Retail $8.99
Value $25.08
Growth  +179.0%
Annual growth 3.0%

1878 Small Bucket

Theme Basic
Year 1991
Pieces / Minifigs 393 / 1
Availability Retired
1878 LEGO Small Bucket
Retail $14.99
Value $71.99
Growth  +380.3%
Annual growth 7.3%

1881 Play Bucket of Bricks

Theme Basic
Year 1991
Pieces / Minifigs 288 / 1
Availability Retired
1881 LEGO Play Bucket of Bricks
Retail $14.99
Value $47.25
Growth  +215.2%
Annual growth 1.9%

1882 Large Play Bucket

Theme Basic
Year 1991
Pieces / Minifigs 449 / 2
Availability Retired
1882 LEGO Large Play Bucket
Retail $19.99
Value $127.91
Growth  +539.9%
Annual growth 5.5%

1978 Santa Claus

Theme / Subtheme Basic / Seasonal
Year 1991
Pieces 39
Availability Retired
1978 LEGO Santa Claus
Retail $3.99
Value $9.81
Growth  +145.9%
Annual growth 2.4%

1979 Snowman

Theme / Subtheme Basic / Seasonal
Year 1991
Pieces 42
Availability Retired
1979 LEGO Snowman
Retail $3.99
Value $13.26
Growth  +232.3%
Annual growth 3.5%

1980 Santa's Elves

Theme / Subtheme Basic / Seasonal
Year 1991
Pieces 30
Availability Retired
1980 LEGO Santa's Elves
Retail $3.99
Value $24.99
Growth  +526.3%
Annual growth 4.4%