Wotlk just has a lot of good raiding that has difficult fights that give a good sense of accomplishment sprinkled all over.
4 horsemen and KT for naxx.
Almost every fight in ulduar had a nice learning curve and beating yogg just makes you feel good.
Twins in toc is decent... No love for that raid really.
Icc is full of cock fights that feel great to progress through, lady deathwhisper is really the only whatever fight.
Marrowgar, gunship, saurfang, fester, rotgut, putricide all great mechanic heavy fights.
Council, blood queen, valithri, sindragosa are all challenging, valithri would be the only fight that errs on being a pain depending on how many healers you have.
The lich king is the pinnacle of the raiding in wotlk, it is hard, very mechanic heavy, full of lore and extremely rewarding both in loot/titles and satisfaction requiring everyone on the raid has to be on their game to beat.
If someone has a lk title you at least know they aren't trash.
Ruby sanctum is another raid that hardly anyone talks about but its just as rewarding as the other fights in icc, trying to pug rs is just damn near impossible. While not exactly hard, it's basically Thaddius with a dps check.