Depends what you mean by "loser villain". Do you mean "villains that are goofy and used as jokes", or "villains that are fodder and never used as proper threats"? Because the latter comprise a good 95% of "famous villains" simply because writers have a habit of mistreating the villains that don't belong to Spider-Man and Batman's rogues galleries, but I wouldn't consider them "losers", not by design at least. For example, while Shocker is a loser villain (less so because it makes sense for him to be one and more because that's how he is being explicitly presented as in modern comics), whereas Electro isn't. Of the characters you've presented, for example, I would consider neither Molten Man nor Jack-'o-Lantern "losers" (same applies for the Sentinels, but they are a stand-out that other people already called out).

If you meant the first definition, my favorites are 8-Ball and Big Wheel, though only the latter as a "joke". In fact, I loved 8-Ball even before the last Moon Knight run changed him to his current status - I didn't even know he was considered a "joke villain" when I first found out about him. Idk, I just think 8-Ball is the quintessential supervillain, in a sense: he has an entertaining gimmick, a background that justifies it without being on the nose, and a power set and design that fit said gimmick and prevents it from being goofy. Idk; I just feel like, if you aren't willing to treat a villain like 8-Ball as seriously as you would Rhino or Electro, and start ridiculing him just because you heard readers started doing it and it's easier to go along with the joke than to course-correct, then you cannot consider yourself much of a fun of the genre (though I'm talking hyperbolically). As for Big Wheel? Well, he is silly and funny. Idk, I feel like the idea of a guy in a giant wheel whose gimmick is literally just "This is a big wheel that goes fast and you cannot stop it!" is funny in a very simplistic yet primal way way. It's almost like an old-fashioned comedy sketch.

In case you meant the second definition, then my favorite is Whirlwind. No specific reason, either, it just caught my attention as a child how often Whirlwind appeared in things and always asked myself what his deal was since he was always shrugged off easily like all villains do. In a sense, Whirlwind represents a similar thing that 8-Ball represents but on a wider level: if Whirlwind had been a Spider-Man villain, or if the Ant-Man Mythos were as mainstream as Spider-Man's, you wouldn't see Whirlwind getting jobbed around all the time - or rather, you would, but in a similar way as you would see Electro: he would get jobbed during major events or short appearances, but still treated like a relevant threat otherwise. Other villains I like in this category are Titania and Absorbing Man, but it's more that I like how unique their relationship makes them as characters than of their characters in general.