The Griner/EDD/Diggins class of 2013 will always be up there, though Diggins was pretty shaky as a rookie before she exploded her second year.
2019 might end up being one of deepest and best draft classes ever (J. Young, McCowan, Arike, Collier, A. Smith, B. Turner, Ezi, KLS in first round; Mabrey, Hiedeman, Carleton, Cunningham in second)--though only Collier, Arike, McCowan were real standouts as rookies.
2008 you had Parker/Fowles 1-2, with Langhorne, Carson, in first round along with other folks who hung around in league for a while. There will (likely) never be another rookie who wins MVP like Parker did.
I might actually give nod to 2006. In top 6 you had Augustus (22pg, 3.8rpg), Pondexter (19.5ppg, 3rpg, 3apg), Currie (10ppg, 4rpg), Sophia Young (12ppg, 7.6rpg), Candice Dupree (13.7ppg, 5.5rpg). (that's 3 of the top 15 all time WNBA Scorers in one draft class).
The 2006 class is often overlooked since all were arguably overshadowed by other legendary players (Augustus overshadowed by Maya Moore, Pondexter by Taurasi, Young's career was relatively short, and Dupree was almost always a 2nd or 3rd option). Insanely top heavy production for a rookie class though that likely will never be matched.
2008 might take the cake though. To expand you had:
#1 Candace Parker- league MVP as a rookie
#2 Sylvia Fowles- only played half of her rookie year, averaged 10.5/7.5 though. Future league MVP.
#3 Candice Wiggins- 15.7 ppg, 3.0 assists, 1.8 steals
#4 Alexis Hornbuckle- major role player for the championship team, led league in steals as a rookie
#5 Matee Ajavon- 8 ppg as a rookie,
#6 Crystal Langhorne- future WNBA All Star and 2nd Team All-WNBA player
#7 Essence Carson-6.6 ppg, future WNBA All Star
#8 Tamera Young-7.3 ppg, 13 year WNBA career
#9 Amber Holt- 6.5 ppg as a rookie
#10 Laura Harper- 5.5 points, 4.0 rebounds as a rookie
#11 Tasha Humphrey- 7.3 ppg as a rookie
#13 LaToya Pringle- future double digit scorer and starter on championship team
#14 Erlana Larkins- 10 year WNBA career, almost averaged a double double in 2014
#15 Shannon Bobbitt- started 17 games as a rookie, averaged 4.1 points, 3.5 assists, was only 5-2
#16 Nicky Anosike - 9.2 points, 6.8 rebounds and 2nd in league in steals as a rookie, future All Star
#22 Allie Quigley - future 3x All Star
#24 Leilani Mitchell - 13 year WNBA career, 2x WNBA Most Improved Player
#30 Charde Houston - 8.8 points as rookie, future WNBA All Star
#31 Crystal Kelly - 7.4 points, 3.3 rebounds as a rookie