The runts of the troll family, having only 5 RHD and being medium-sized. Then again, less HD tends to be a good thing in the context of this thread.
Forest trolls are some of the smartest trolls out there, being roughly as intelligent as a human. Their physical stats aren't bad (+6 strength and dexterity, +10 constitution), they got weak 1d4 claws and bite, and they have a low climb speed.
The trolls also have a badly-worded Poison ability. It *seems*, but isn't outright stated, that given some time forest trolls can coat their claws and manufactured weapons with a (presumably free?) poison that deals 1d6 constitution damage. That's not bad at all, but it wouldn't hurt for there to be some clearer wording.
Finally, because they're trolls after all, forest trolls have fast healing 5. It's definitely useful, and at low levels even has some notable in-combat effects.
Is all that worth 5 giant RHD? I'm not sure. In terms of damage, the forest troll will find itself outperformed by a lot of monsters, or even some PC races. Toughness-wise, it probably beats many of those, but 'being tough' isn't a combat role. Being a walking poison factory seems like it may have some utility, but it's important to note that nearly-equivalent poison can be produced by a caster with a viper familiar.
For now, I'll assign -0 LA, though again this is a relatively playable -0. Two lost points of BAB and unimpressive strength keep it from entering +0 LA territory. Remove one or two HD and this should be more balanced.