To be honest, I have always found 3e giants a tad bland. Take a base giant, slap on a few extra HD, improve his stats a little, maybe give it 1-2 extra abilities, and you have a new giant variant. It was only with the advent of MM3 that we actually received some interesting giants with fun abilities.

Another personal gripe, why bother giving giants a ranged attack when they suck at it so much, given their crap dex scores? If you want them to hurl rocks with some degree of accuracy, at least give them the brutal throw feat, letting them apply their str mod on attacks instead of dex.

Looking at the MM2 giants, I am having trouble understanding how they received the crs they did, considering they are only marginally better than their MM cousins. One thing to note: They don't have a penchant for wearing armour, which is why their ACs are pretty much the same even though they tend to have better dex and NA. If you opt to equip them with simple armour, the boost in AC may make them tougher. A simple breastplace already adds +5AC after all.

Anyways, my recommendations are as follows.

Forest Giant: Basically a hill giant, advanced to huge size, with poisoned arrows that will likely never hit due to its crappy +8 attack bonus. MM2 gave it cr11, I would give it cr8 or 9 max (the MM guidelines recommend +1cr to a size increase+stat boosts).

Sun Giant: A reverse frost giant increased to huge size (with accompanying stat bonuses) is all I can say. I would peg cr at 8+1(huge)+1(SLAs) = cr10, down from its original cr of 12.

Ocean Giant: Perhaps there is some premium for facing underwater, but as it stands, this beast is simply a storm giant (cr13) who also happens to be immune to blunt weapons. This is definitely no cr19. I would place him at cr14 tops.

Mountain Giant: This monster is infamous for being notoriously weak. At cr26, his pitiful will save of +10 means he will almost assuredly fail the first dominate spell that comes his way (Any self-respecting wizard can manage a save DC of at least +30 by then). Not to mention he has no defenses like immunities, dr or sr. +30 attack isn't that strong either, especially for a bruiser who is expected to power attack some. It seems here that being colossal may be more of a hassle than a boon, considering the massive attack/AC penalties it bestows.

And why is his str only 43, when a huge giant already has 39str? By extension, his str should be at least 50. I would peg it at no higher than cr16, and even that is a tossup because at this point of the game, you don't really go far as a dumb melee brute with no magical support.