I think your answer also is determined by what you consider to be 'competitive.' Is 'competitive' having a shot at winning? Making the top half percent of the division? Making top 3? Are we talking which division is hardest to field a team in? Or are we assuming you could make any team and which would be hardest for the first 3 questions (winning, top half, top 3)?
Winning: Large Senior, Large Coed, International, Sm Coed, Sm Senior
Top Half: International Coed 5
Top 3: Large Senior, International Coed 5, Sm Coed
Why these ones? Looking at last years Worlds, the top 3 for large senior are VERY well established in their top 3 spots. Spice and Tsu have done an EXCELLENT job in bringing some competition to a very repetitive division. Large Coed is hard to field and has some VERY strong coed programs that are hard to shake. International in GENERAL is a tough division because you're constantly being cut and even then you only get top 3 to finals. Never mind the fact that there are some RIDICULOUSLY strong programs leading that division. Sm Coed's scores at Worlds for top 2 were leaps and bounds ahead of everyone else, from what I remember. Getting that elusive top 3 spot is a challenge when the top 2 are SO strong. Small Senior has such a deep division that getting anywhere decent in it is a huge feat, never mind beating a multiple world champion for the top spot. Every mistake in that division counts HUGE.