Well going by what is on Blizzards own site
http://us.battle.net/wow/en/zone/#expansion=0 Vanilla had 20 dungeons and 6 raids bearing in mind that of those 6 Ruins of Ahn'Qiraj was the only 20 man raid at the time
http://www.wowhead.com/zone=3429/ruins-of-ahnqiraj all the others were 40 man..
TBC had 16 dungeons and 8 raids with Karazhan and later Zul Aman being the only 10 man raids..
WoTLK had 16 dungeons and 9 raids though in all fairness there were only 7 unique raids as Naxx and Ony were rehashed Vanilla raids..
Cata had 14 dungeons but only 10 of them were unique as you had Deadmines and Shadowfang Keep rehashed, and Zul Aman and Zul Gurub converted from raids to dungeons.. And only 6 raids..
MoP had 9 dungeons but only 6 were unique as 3 were redone Vanilla dungeons and it had 5 raids..
As you can see that while Vanilla had the most dungeons, BC had the most raids followed closely by WoTLK and Cata.. And when I mean that BC had the most raids that is most new/unique raids and not rehashed ones..