To determine this question, we just have to search the prices of frames in the shop wiz, since every member is given a frame and the more the frames, the more the members, the cheaper the frame. Therefore one can *almost* conclude that the frame that is the cheapest should be the most popular team.
These are my results
Altador Cup Brightvale Frame -- 4k
Altador Cup Darigan Citadel Frame -- 700
Altador Cup Altador Frame -- 1k
Altador Cup Faerieland Frame -- 1.8k
Altador Cup Kiko Lake Frame -- 10k
Altador Cup Haunted Woods Frame -- 1.2k
Altador Cup Krawk Island Frame -- 500
Altador Cup Kreludor Frame -- 2k
Altador Cup Lost Desert Frame -- 1k
Altador Cup Maraqua Frame -- 900
Altador Cup Meridell Frame -- 4k
Altador Cup Moltara Frame -- 1k
Altador Cup Mystery Island Frame -- 2k
Altador Cup Roo Island Frame -- 2k
Altador Cup Shenkuu Frame -- 900
Altador Cup Terror Mountain Frame -- 1.8k
Altador Cup Tyrannia Frame -- 4.5k
Altador Cup Virtupets Frame -- 2k
Of course, we simply can't deduce the winner based on the number of participants in the team, and one cannot simply neglect the possibility of frames present in SDBs. Also, there might just be that powerhouse joining Tyrannia with 50 accounts and SS-ing from there, pulling a shocker no one expected. Nonetheless, shop stats can (hopefully) bring out a glimpse of who can win the altador cup (Krawk Island FTW)
This results are of multiple wizard searches, though not complete market searches, and all anomalies were discarded before calculation.
Hope this can help shed some light on those who haven't chosen a team, though it's 2 hours from registration closing.