I don't think I need explain what hydras can do. They've got plenty of bites (all of them usable with a single standard action or charge), a lot of fast healing, and some reasonable stats. Hydras also all have an improved version of combat reflexes for free, which increases their number of AoOs per round to their head count (and possibly even more than that). The latter is especially interesting when taking into account their natural reach.
Enemies chopping off heads may seem like an obvious weakness, but it really isn't. At most, people are wasting an attack of theirs to maybe reduce your number of attacks per round by 1 for a few rounds: that's not worth it, action economy-wise. Even the smallest hydra PC will have 10 heads most of the time: killing it by stabbing is probably faster.
Note that five minutes of preparation and an ally with a knife let a hydra double its number of heads for the day. That means more attacks, more AoOs, perhaps even more breath weapons if we're talking one of the elemental variants.
Every extra hydra HD adds one head (effectively two), more fast healing, some natural armor, and occasionally a strength bonus.
+2 LA seems reasonable enough for the weaker varieties, but I'm not quite sure if HD advancement outlined above is as valuable as a PrC level or initiator dip. Then again: reducing any of the hydras' LA to +1 means the one directly before it becomes pretty much worse in every way.
For now, I'll keep hydras at +2 each (because why not do the reasonable thing for a change?). People who have opinions on these tricky beasts are free to share them.
Pyro- and Cryohydras
About equal in power (with pyrohydras stronger defensively and cryos offensively), these variants get an elemental subtype and a very potent breath weapon that deals dozens of d6s in damage. I'm going to assume it's a standard action to use (implied heavily, but not stated outright) and add +1 LA to each elemental hydra.
Of course, this assumes one doesn't go with the interpretation that a ten-headed hydra would breathe ten jets dealing 30d6 damage each, because things start getting even more bonkers then.