Ditto, Wobbuffet, Shedinja, Shuckle, Castform, and Smeargle are all the worst.
I have to disagree with Castform's inclusion on that list, speaking as someone who used one in-game in Ruby. I still have that Castform to this day. It's gimmicky, yes, but it has balanced stats, great coverage, and its weather moves can help out the whole team. And Wobbuffet... I never used one in-game, but I did use a Wynaut, and Destiny Bond is why I was able to take out Norman's stronger Slaking. I won't deny it's tough to train, but it's not like it's not worth the effort, at least in my experience.
I would take ditto off the list regardless of whether or not it has imposter because it's definitely not THAT bad.
Unown, Ledian, Dunsparce, Farfetch'd, Luvdisc, and Delibird
I'd have to vehemently disagree with you as far as Dunsparce is concerned. In fact, HG/SS is the best game in which to use it, because you can catch it early, and it'll be able to learn Headbutt through the move tutor. Get one with Serene Grace. Make sure it learns Glare and Headbutt. Now stand back and watch the carnage ensue. Serene Grace gives Headbutt a 60% chance to flinch, and even when it doesn't, the target still has the paralysis to contend with. Put simply, they'll be sitting ducks more often than not. I used a Dunsparce in this manner in HeartGold, and I don't regret it in the slightest.
trubbish, who will get that pokemon,its plain garbage....
Ha ha. As it just so happens, my Trubbish in Black 2 saved my *** during the PWT, and as a Garbodor, thrashed the champion's Aggron. Breed one so it hatches with Rock Blast and maybe Gunk Shot.
The Normal types you encounter at the beginning are bad in-game too, like Raticate, Furret, Linoone, Bibarel and Watchog.
Furret and Watchog especially, have bad stat distributions.
The first Bug types too. Beedrill, Butterfree, Ledian and the sorts.
I can't speak for Furret, Watchog, or Ledian, but as far as Beedrill, Butterfree, Raticate, Linoone, and Bibarel, I couldn't agree less. I'll admit Beedrill is a far cry from the strongest Bug-type in the game, and I will concede that it doesn't get a lot to work with at first, but you get it to learn Twineedle, and you will start to gain respect for it. I was able to solo Lt. Surge in FireRed with a Beedrill. It wasn't even overleveled. STAB Twineedle is nothing to sneeze at. And Butterfree... Oh, don't ever say to me that Butterfree is bad in-game, because I will prove you so wrong that your grandchildren will be apologizing to me. Try using one in FR/LG. As soon as Metapod evolves, BAM! Learns Confusion. Earliest access to a Psychic move you will ever get in that game. And then, a few levels later, Butterfree learns perhaps its most useful move- Sleep Powder. Couple that with Compoundeyes, it's all Spore is hyped up to be, without the hassle of raising a Paras. You use it properly, Butterfree will serve you well throughout the game. I actually took out half of Blue's team with a level 50 Butterfree. When I battled him as the Champion. His team grossly outleveled mine, and Butterfree was still able to topple Alakazam, Exeggutor, and friggin' Blastoise. Raticate? Its stats aren't the best, but they serve it well. And Rattata learns Hyper Fang at level 13- that's essentially 120 base power at level 13. I've used Raticate to great effect many times before. Linoone? I trained one in Ruby, just for Pickup. Still have that Linoone, in fact. It's fast, it hits respectably hard, and it's great for carrying bothersome HM moves. Strength in particular is good both in and out of battle. And Bibarel isn't bad either. Strength and Waterfall make it very functional, both in and out of battle. Besides, it's one of the earliest Water-types you can get in Sinnoh. Saved my bacon against Roark more times than I can count.
Try Watchog, Basculin, Garbodor and Swanna. And depending on how easy the in-game trainers are, imposter ditto is still worth considering.
Basculin? Are you kidding me? See one of those things get off an Adaptability-boosted Aqua Tail and then come talk to me, because trust me, I speak from experience when I say that it destroys things. Utterly.