It's interesting to consider this one, and then compare it to the forest levels of EotB 3.
The forest level in EotB 2 (also the starting level) is small and grid-based, and yet packs in some interesting things. There's a graveyard squeezed into a corner, and a secret passage can be found by exploring the 'walls' closer. But it also accomplishes something else despite its small size - it's confusing to navigate. Short passages, plenty of turns and no real landmarks around. It's actually not one of the best places in EotB 2, but the fact that it comes in at the start, and is dealt with so quickly due to its size, means that it doesn't leave a bitter flavor in anyone's mouth.
Meanwhile EotB 3 has two forest levels, each with a different theme. The first one (which also serves as the starting level) comprises of a large, open space to the west, and then a bunch of tree 'obstacles' to the east, which need to be chopped down to be removed. Exploring large open spaces in a grid-based game without an automap is tedious because the chance of missing out on something is actually quite high... but at least it's easy to find your way again if you get lost. The choppable trees are only a speedbump - forcing the player to do tedious busy-work for little to no reward - though it is required to find the exit gate and progress further. The end result is that the first forest level's novelties wear off really quickly, and instead end up annoying and delaying the player.
The second forest level is no better. It's a long, winding tunnel filled with briar patches which split off into different directions. Due to how they function, diligent mappers will run into a bad time here. Add into the fact that there are "stubborn trees" here which need special methods to be removed and invisible monsters, and most players just want this section to end already.
For further examination, consider this; the devs of EotB 2 went on to make Lands of Lore, while a different team handled EotB 3. What are the forest levels in LoL like? Long, winding tunnels with plenty of turns to make navigation difficult. Thankfully the automap saves the day, as well as the game rewarding aware players who peek into every crevice and hole they come across. The only notable part of the forest IMO is the swamp, and how the Freeze-spell can help you navigate it.
Finally, to complete the chain of grid-based forest levels, there's the Czech RPG Cardhalia which is heavily influenced by Lands of Lore. It too features forest levels, but it's the one after the mines that I want to mention. Imagine creating an completely empty map, then adding in Tetris-shaped blocks around the map, making sure they never/rarely touch each other, until you've filled out the place. The end result is an incredibly annoying level to traverse, it gathers up almost every negative aspect of the aforementioned forest levels, and none of the positive ones.
In short, I don't recommend any of these as 'best forest level', but they're still worth discussing.