What are some good Terran counters to Hydralisks?

I was surprised how strong Hydralisks are against siege tanks - TeamLiquid tells me that tanks are actually strong against Hydras.

But why? Tanks do 35 damage (+splash) in siege mode to hydras. But their rate of fire is very low. Thor isn't any worse, and also provide air protection. Marines (with Medivacs) should work even better.

So questions is:

1. What is really good counter to hydras?
2. If tanks, then how to use them properly?

Edit: (after Tzenes's answer)

Pure hydras are rare, usually if you go pure marines eventually you will get Banelings. So you could use some marines, but don't think a lot of them is very good, only if you are able to make them in unexpected manner.

Regarding tanks: shouldn't Thors be much better than tanks against Hydras? They are almost twice more expensive (300/200 against 150/125) and has a little bit longer cooldown (1.28 against 1.04), but has 2.5 times more HP (400 against 160), 60 ground damage against 15 (in normal mode) and provide good anti-air. It is even better if you consider splash damage in siege mode too.

What do you think of that?

So, the short answer to your question is: Marines and Siege Tanks.

Let's start by looking Siege Tank against Hydras as compared to Siege Tanks against Roaches.

Assuming no upgrades, it takes a Siege Tank 3 hits to kill a Hydra (35*3 > 80) and 3 hits to kill a Roach (50*3-3 > 145); however, with one armor upgrade it takes Siege Tanks 4 hits to kill the Roach. Since they're both the same size, this means Siege Tanks are better against Hydras than Roaches (even better since Hydras represent more resources). Despite their longer range Hydras also move much slower than Roaches giving Siege Tanks much more time to hit them with out repercussions.

So the question becomes, how do you use them better?

The key principles to Siege Tanks are:

• Don't clump them together (the time it takes to move from attacking one Tank to the next matters)
• Layer them (while fewer will get earlier attacks, this means the enemy must move through earlier layers to attack later ones)
• Don't leave them alone (despite their high hp, use Marine/Marauder to soak damage)
• Use spotters (Siege Mode range is actually longer than their sight range so use an Air unit or near by tower or move a couple Marines forward).
• You need about 6-7 tanks to be effective. While lower numbers will still help it takes at least that many before you can move out
• Abuse chokes and high ground (this should be obvious)
• Don't move on to Creep (always use Scans/Ravens to kill Creep Tumors, you don't want Zerg covering that ground faster).

Strangely enough, Marines are also amazing against Hydralisks. Because most Hydra forces are lead by Roaches (which are good vs Marines) its easy to overlook that, for cost, Marines both out dps and out tank Hydras. Since a Hydra is 100 min, 50 gas we can say is the equivalent of 2-4.5 Marines (depending on how you count gas), however they have half the cost effective dps of a Marine and less hit points than 2 Marines (not counting Stim or Combat Shield). In short, Hydras are overpriced Marines.

If you're up against pure Hydra do not hesitate to Macro Marines.

Regarding your edit about thors vs tanks, thors are definitely easier to use and except for a failed thor drop it's almost impossible to screw up using thors. However as much as thors annoy me because they're powerful and easy to use, there's little that scares me when playing as Zerg more than well played tanks.

Like @Tzenes said, you really need a decent sized force of tanks to be effective. 6-7 sounds about right for an average sized encounter (obviously you'll need more if you're going up against a 200/200 maxed army). The key is that you need enough tanks to 1-shot the front line of an approaching army.

Sure if they do get up in your face thors are better than tanks, but the key with tanks is to have enough tanks to kill them before they can even get in that close. MM in the front help slow down the advance while your tanks pick them off, and medivacs serve a dual purpose, both keeping the MM in front healed, as well as giving vision to the tanks to shoot further.

Do not underestimate the power of tanks range. At 13 (11 if you don't give it more vision) that's more than twice the range of upgraded hydras.

As far as I can tell trying to take on an MMMT force head-on assuming both players have been macroing equally well is suicide. The zerg player will need to abuse their mobility and attack them somewhere else forcing the terran to split up their forces or attack from all sides (main army in the front, zerglings and/or mutas around back and really get a good surround)

Long story short, tanks get exponentially more powerful as you get more of them until the point where you're killing things before they can even reach you. Thors are more powerful as a single unit, but adding more doesn't add as much power as adding more tanks.

Tanks are great against hydralisks because hydralisks are size: small. They clump together in a similar spacing as do marines, so 1 siege tank can hit, I believe, 9 hydras with a single shot. Layering and protecting the siege tanks is paramount. If the tanks have enough time at range, they can kill huge swarms of clumped hydras.

For that same reason, Hydralisks are good against Thors. Thors slightly overpower hydras on a resource per resource basis, but hydras in packs can turn that around quickly. Once you can get some clumping, they take thors down faster than any other zerg unit.

Yeah have about 8 siege tanks in a choke point with MMM in front. Maybe a couple of widow mines too for any banelings.

Just be ready because if you wipe their army a good Zerg player will be sure to follow it up with Blood Lords and Corruptors. So I'd recommend a quick counter attack with the MMM while making Vikings.