You have to look at the quality of players they played with year by year. Comparing their freshman teams, you have:
DT-she had a ton of talent on her team but they mid-season lost a strong POY candidate in Svetlana Abrosimova, then they lost Shea Ralph who was a strong contributor as a RS-senior (averaged 11ppg). That was a huge blow. By the 2nd half of the season the team only had 1 player who would make an AA team, and that was Sue Bird on 3rd team. Sue Bird back then was not the Sue Bird she would be come.
Even more than that, the level of competition in WCBB that year was also at an all time high, as the incredible 2001 class were seniors and the loaded 2004 class were freshmen. The level of competition was deep and strong, with probably 7-8 teams all having a realistic shot of winning it all. DT famously came up short against ND with a dreadful shooting night, plus Bird was outplayed by Ivey and ND went on to win.
Moore-she had a talented roster, but Charles and Montgomery were not nearly as good in 08 as they were in 09. Despite somehow making an AA team, Renee shot 37% from the field and 31% from deep. She didn't have strong decision making or shot selection skills needed for UCONN to win it all. Tina Charles was inconsistent and in Geno's doghouse for much of the year. She clearly had potential but didn't start to put it together until her junior year where she took off after that. Back then she wasn't the consensus top player in her class. Maya Moore was far and away the team's best player, averaging 17.8 points, 7.6 rpg, 3 apg and shot 54% from the floor and 42% from deep. Also, the competition that year was particularly strong too. 2008 had 7-8 really really good teams capable of winning it all including Tennessee, Stanford, Rutgers, Maryland, UCONN, LSU and North Carolina. And Moore led them to a 36-2 record where they ran through several of these teams and entered the NCAAs at the #1 overall team. They came up short in the Final Four, but Moore had a strong game with 20 points and 9 rebounds. Montgomery was probably the biggest reason for their struggles that game, as she shot just 4-18 from the floor and 1-9 from deep.
Stewart-she came in and joined a roster with everyone back besides Tiffany Hayes from a Final Four team that lost in overtime to ND. Stewart had a very up and down freshman season. Spent time in Geno's dog house and had moments of brilliance and moments of really struggling. Keep in mind, back then a lot of people on this board were praising how good Tennessee's Bashaara Graves was in comparison to UCONN's freshmen. Funny to think about now.
There were a lot of moving pieces on the roster, but ultimately it was a loaded group of players, and they were very good. Stefanie Dolson started to really put it together as a junior and earned 3rd Team AP-AA honors, while KML broke out as the team's leading scorer and was a 2nd Team AP-AA while shooting 49% from deep. She also had Kelly Faris as a senior who was the ultimate glue player for UCONN and likely was in consideration for an AA team. Point being, UCONN was immensely talented and legitimate title threat in 2013 even without Stewart. In the Baylor game, UCONN led much of the way and lost a 6 point hard fought battle with Stewart playing 7 minutes and not scoring. UCONN also choked away a triple overtime loss to Notre Dame with just 5 points and 1-7 shooting from Stewart. Once Stewart found her stride, UCONN transformed and dominated the competition, with Stewart being the standout star in the NCAAs.
Also worth noting, there were only 3 title threats that year in Baylor, UCONN and Notre Dame. The three of those teams only had 2 combined losses to outside competition, and it was Baylor losing to Stanford with Sims missing, and the biggest upset in NCAA history when Louisville stunned Baylor. The competition was not deep that season.
Which season of the 3 is most impressive? I'd say Moore's, hands down, even with Stewart stepping up on the biggest stage. But that's up for debate depending on what you value most.
Now, if you look at their sophomore years...all 3 have similar resume's although DT's is probably weakest since she was only a 2nd Team AA and not a POY like Moore/Stewart. All 3 teams went undefeated. All 3 had multiple AA teammates (Cash/Bird, Montgomery/Charles, Hartley/Dolson). All 3 were excellent.
Fast forward to their junior years. All 3 again are comparable since they won a title and earned POY awards, but DT probably takes the cake here going 37-1 with a young roster and sweeping all the POY awards. Moore split POY awards with Charles in an undefeated campaign, and Stewart won almost all POY awards but didn't have an undefeated season. Moore/Stewart were playing with other AAs (Jefferson/KML) or another POY (Charles). Not a lot of differentiation between these three this year, but DT's route was the hardest so she gets a slight edge.
Senior year I'd give Stewart the nod for having the best season. Moore's was spectacular even though they were upset by ND and came up short. DT's was honestly underwhelming until UCONN went on to win the whole thing. That said, Moore and DT didn't play with any other AAs. Stewart played with the likely 2nd best player in the country in Jefferson and a 2nd team AA in Tuck. Still, I'd give the nod to Stewart.
Add all that up and what do you have? Not a lot of differentiation between the 3. Ultimately like I said, choosing the best comes down to what you value. Is it championships? Winning in the Final Four? Individual stats? Being able to carry a less talented team? Any of those players you can build a strong argument for based on their credentials and resumes. The fact that Moore's biggest negative is coming up short in 2 games where she scored a combined 56 points shows how little difference there is between her and Stewart or her and Taurasi or her and any of the other players I mentioned.
So in summary, while I think Stewart has a strong case for being the best WCBB player ever, so do several other players so it isn't cut and dry.