Hi there! I'm new, so don't troll too hard.
Not a WC3 expert in any way, shape or form, but I do know a thing or two.
Kay, gonna run through some of the nominations for worst in here so far, then give my worst three. TL;DR is at the bottom, naturally.
PotM = Has best scout in game. Aura is beast for T1/T2 play. Searing arrows is good for hero focus, and her Ulti is imba with AM pot. She has decent DPS, and relatively good armor, but the low HP makes her squishy, and a bad choice against nuking heroes. I do feel, however, she is perhaps one of the best counters to a DH, as she is still effective without MP.
KotG = While he may be easily countered by dispel. Roots is great for hero focus. Treants are useful for creeping, and creating paths through trees. Thorns is great in a NE mirror, and can give your bears that edge if opponent choses a different 2nd hero (usually Naga). Again, Tranquility is great if you have AM pot. Good 2nd hero for fast T2 build. However, very squishy to focus, and doesn't have the MP for both roots/treants without wasting MW's.
Dreadlord = Best used for fast expand, with a later tech to gargs, DK 2nd. Sleep surrounds are great if you're against a newbie, or even taking that APM away from a strong player can help in the long run. Carrion isn't the best AoE in the game, and I rarely get it unless I see mass T1 before I've teched a 2nd skill, but the aura works well in a ghoul/garg build which is common with this hero. Ultimate probably best UD Altar hero ultimate, as it isn't channeling, and doesn't require a lucky break with dead unit composition.
Alchemist = This is a very slandered hero, to say the least. However, his heal is completely underrated, bare in mind, it is most useful when playing against (or even with) massed range. In most matchups vs UD, you'll be facing a high fiend count, with DK/Lich. or early game against a NE DH/archer/hunt masser. Cast it correctly, and you won't heal any opponent units. High APM and good micro is essential for this. His armor reducing debuff is useful in several situations - It can counter a scroll of protection/Inner Fire/Devo aura. Used in conjunction with Faerie fire, it can absolutely destroy armies in seconds. His own low armor makes him squishy though, with low damage reduction. His ultimate is one of the best in game. Remove a unit from battle, AND GET PAID? Very useful in end-game 1v1 when map is mined out. Or when your opponent is using high food units like Chims/Wyrms.
Crypt Lord = What he lacks in speed, he makes up for in tankabilty, just like the TC. Beetles are great scouts once they get burrow, but dps and tank-wise, better to use a skelly staff before the skill is lv 3. His passive is great. Makes him quite tanky, and with DK 2nd, little micro is really required to keep this guy alive. Impale does low damage, but against mass ranged (archers/rifles/casters/fiends/HH) that often line up, one impale can change a battle. His ultimate is OK, not the greatest, but worth getting, for sure.
Pit Lord = Cleave is very useful against T1 massers, Howl of terror is great against high DPS units, usually T3 melee/heavy air. Rain of fire demolishes worker lines better than any other AoE (except, arguably lv3 FoK and Pandas haze+fire). Pit lord is IMO the best counter to a human tower push because of this. Combine this AoE with slow, Blizzard, and Flamestrike and you can force an early T3 GG. Naturally his Ultimate is 100% worth getting. Similar effectiveness to the DL. Combining stomp with ROF can be just as deadly as Carrion + Infernal summon stun.
Honourable mentions:
Naga Sea Witch = Perhaps I dont know how to play her right. In fact, I'm sure I don't know how to play her right. She's most often used at T2 for NE, or T2/3 UD pressure push/nuke, but her arrow is about as effective as the Dryad's passive, She's squishy and forked lightning pales in comparison to breath of fire. Her ultimate is like a NE DOT build, and the mana shield skill..well...have you ever seen a pro use it?
I play random 1v1, no FFA or RT, so I'll mention my most used tri-combos for each race in 1v1:
HU - AM - MK - Pala (lv2 AM before MK)
NE - Warden - DH - DR (lv3 warden before DH)
Orc - BM - SH - TC (1 grunt tech, fast t3 for that TC)
UD - DK - Lich - DR (TED fiends build, 2 fiend tech, straight to t3)
Naturally depends on the map, sometimes I go solo DR w/black arrow in a small map for early worker harass, NB: useless vs NE.
I do love to use the unexpected combos though. DR's Life drain is a real hero killer if you get a stun/slow off and the LV3 drain. Bolt/Roots/Stomp/Impale. Also, this imba move - MK bolt + Bloodmage/DR drain combo.
Whoever it was who asked about Phoenix being the only Flying summon - Beastmaster's Hawk and PotM's Owl scout. However I would argue that Phoenix is on a par with PL/DL's ultimate summons. And is the best flying summon available.
The effectiveness of each hero depends on the map. A warden is useless if she can't get to level 3 quickly, but IMBA if she can creep to 5 for that lv3 FoK. TS is a great example of a map for odd hero choices because of that fast creeping ability, MK is often picked first for HU because of this, and DH is only really used in a talon build vs orc or NE mirror.
On topic, the question was 'Which do you think is the worst and/or weakest Warcraft 3 hero?'
I'll answer this with a top 3.
I guess my first choice for worst would be the KotG. At T2 he can be countered by every race but UD. But because T2 UD can nuke his low base HP so fast, you dont even need the dispel to get him to retreat to his moon wells. If I see a KotG coming at me, I know to tech to a few priests/dryads/S.walkers or get a shadow priest depending on map. His DPS is low, too. I only really fear his ultimate being used with an anti-magic/invul pot. Thorns? Ok I'll go ranged. Countered.
My second choice would be the Naga. This is just personal opinion here. I can't use her. She dies so insanely fast to focus, and with only a 50% usefulness rating on her skills she's only 1 skill better than the KotG. Sure, her DPS is OK, but she's so mana intensive with those arrows, MP pots/clarities are a must, with HP pots/regen scrolls. She's just too high maintenance, for such little reward IMO.
My third choice is tricky. But I gotta give it to the Alchemist. His low armor makes him an easy target for an agility hero. His heal requires too much micro for the average player to make it worthwhile. Chemical rage is good for that DPS boost and hero focus, but you only usually tech two skills before lv8. Acid bomb is his strongest skill, and is anti-army, so you're kinda forced, into chosing to be a hero killer or an army killer. Each race has a hero which is better at both of these. His ultimate is so rarely seen, but can be countered by using low food units, or an early silence/stun.
For my opinion on strongest hero in the game, I don't honestly think there is one. BM, DH, and warden are possibly my top three though. Highest DPS, best hero counter, and most useful skillset.
Dark Ranger is my favourite though. Dem bewbs.
TL;DR
KotG, Naga and Alchemist. Sure, they're all useful in certain situations. But there are fewer situations these three are useful in, than most other heroes. This is all personal opinion of course, and i'm likely to get stick for the Naga vote, but remember I'm not a pro by any means, and how many times have we seen Moon go Naga 2nd? lol.