Most Feared Unit - Wood Elves

No, in all honesty, I think one of the most feared units (for me at least) would have to be multiple units of Wild Riders. They are definitely a glass cannon, but can hit like a ton of rocks! They're also very hard to catch and can jump around the board to threaten your backfield fairly quickly. Back them up with a Glad Captain on Great Stag (or Great Eagle), and you have a force that you can't ignore.

I don't know that there is really any ONE unit that I'd say is overly scary since practically all of the army is extremely squishy or has built-in weaknesses (like Flammable). Treekin could be be pretty nasty, but seem to fall just short on the "OH CRAP"-O-Meter. Wardancers and Wildwood Rangers are about the same in that they need to be able to hit quickly and decisively. Waywatchers work better on paper than on the table top, IMO. They either get pushed to an outside flank where they have to fight for position to stay relevant, or you have to devote way too many units to defending them, and in a very squishy army, that's usually not a good idea.

IMO, Wood Elves are, and should be, played as a "hit and run" army. Stack your ranks with fast, mobile units that can threaten weak points in your opponent's defenses, and let your superior ranged units pour on the pressure from afar. Add in a few ambushing units (like Glade Riders), excellent redirectors (like Great Eagles) and other tricks in the WE arsenal, and you have a potentially NASTY army. But they all need to work in tandem.

FYI - One of my favorite builds (if I'm feeling particularly unfriendly) is to take the Acorn of Ages on a Spellweaver, and the Moonstone of Hidden Ways on a Glade Captain. With any luck, you'll be starting the game with 4 woods that you can add to the terrain (or any existing woods) and provide you with even more immediate board control. Take Calaingor's Stave as well, and cast the spell "Tree Singing" on an enemy unit within a forest, and do immediate damage, or keep them surrounded by the woods that you brought along with you, forcing dangerous terrain tests, all game long. I have utterly demolished armies using this tactic, weakening them to the point of breaking, only to have a unit of 30 Wildwood Rangers suddenly appear in a forest behind them (that they just made it out of), and a unit of Treekin in front of them, all the while being peppered by arrows. That's rough. I usually feel pretty guilty after playing that build, so I try not to do it very often.

I play Brets normally, never really been scared of Woodies.

So I say "none".

When playing dwarfs though, anything that shoots!

And when playing High Elves I always put it down to the roll of the dice, hehe.

Not enough shots to worry about, Use terrain, target saturation - push forward for pressure etc...

I'm usually in combat and I don't have to worry about being shot.

I don't use the Lance formation when fighting woodies, I just go in 1 line - very hard to flank / avoid - but as they're only elves I can easily win combat.

I use life magic too, so regrow some knights.

The first units to be targeted, almost always are - Trebs and Pegs.

I always put 1 treb in each corner (that's right, I use them as chaff - they never hit anyway!), meaning the usual 2 units of WW are next to them...
Resulting in my knights not taking hits.

Maybe I have too much experience vs. Woodies, but I've only lost against them once (3 great eagle riding heroes, a treeman and a lord on a dragon).
It was such a peculiar and maneuverable build I couldnt use my tactics...

I agree hit and run is best for elves.
And I find that easy to beat with Brets.

Yet to try them with my ogres though.

While Woodies have lots of good units, they don't have a stand-out one that opponents absolutely hate, like WoC Daemon Princes. It's the combination of them all running around the place, pincushioning you as you struggle to pin them down that's frustrating.

If I had to pick a unit, it'd probably be the humble Glade Guard, filling up your opponent's core allowance with a unit that will pour death on you from afar, with an extremely versatile selection of magic arrow upgrades. Of course, Deepwood Scouts are more manoeuvrable, and Waywatchers are nasty, but neither are core, so aren't nearly as numerous.

Alternatively, if you're fielding an army with army-wide Fear and minimal ranged threat (VC, Daemons) then those opposing Wildwood Rangers are budget Swordmasters.