I think all three are viable.
A different question to ask: What else can a tank-spec'd class do in raids?
[Specifically, many encounters require multiple tanks, typically on multiple targets (though there are some that several tanks have to fight for agro on the same target, like Broodlord, pre-BC).]
Feral Druids are great for melee DPS, and therefore make great off-tanks insofar that once their target is dead, they can pop into cat and contribute non-trivially to the DPS, far more so than a Prot Warrior or Prot Paladin.
Protection Paladins are great for off-healing (mana-permitting), once their target is dead. And for encounters like Hydross, you almost need one to AoE tank the adds. Improved Holy Shield also makes them well-suited to tank fast-hitting, dual-wielding bosses like Moroes, Romulo and Prince. The ability to front-load agro in "DPS-race" encounters also is nice.
Protection Warriors, as mentioned above, can't do much else other than tank. A DPS Warrior is better suited to off-tank than a Prot Warrior. But they tank really really well, with the stamina of druids and the shield blocking of paladins (to push crushing blows off the combat table), and is much easier to gear up, as said above.
Long story short: All three can main tank, and I've seen them all do well first-hand, at least all the way through Karazhan and Gruul's (in-person). There are tons of videos online of all three as Main Tanks all the way through SSC/TK/MH/BT.
Their non-tanking duties, on the other hand, greatly differ. If a group/raid only needs one tank, I'll take any. If it requires more than one, I'll try to take a mix.
Different strengths. Different weaknesses.
Different flavors, same role.
EDIT: To answer your question, Thul, pick whichever one is more fun to you. All three are fun in their own ways. A druid is not a paladin is not a warrior. They all play very differently. They tank differently.
Do you like variety or focus? Do you like to re-spec often? Do you like the option to heal (Paladin/Druid)? Do you like to wear plate and tank with a shield or DPS with a nasty 2H hammer (Warrior/Paladin)? Do you like to stealth (Druid)? Do you like to cast spells (Druid/Paladin)? Etc etc etc.