Nothing original in my opinions, other than using adjectives other than "worst"!
Blofeld in YOLT is the most "disappointing" villain - I've never seen all the fuss over Pleasance's portrayal. To me he's a short, mincer of man, who does very little other than occasionally raise his voice. He's weak because IMHO, there's no sense as to why he's doing what he's doing other than he wants to. Goldfinger, well, he's a greedy SOB; SPECTRE in both FRWL and TB exist as an organization with a master-plan. Blofeld in YOLT, just seems to be doing it because, well, because that's what the plot requires. Poorly written, and poorly executed. Not to pick on Pleasance, but his "President" in Escape From New York comes across as weasselly, manipulative, cowardly etc. His super-villain in YOLT, is nothing more than a short-man with a cat.
"Non-threatening" villain would be Kamal Khan from OP. Ultimately, a not altogether unlikeable chap who just happens to do some smuggling.
The "least-convincing" villain might be the trio in TLD - always a conumdrum. Between the three of them, you've got one great villain - a physical presence, a bit of a nut/egomaniac, and someone with some suaveness and a little humour. Meld them into one person and you might get someone like Silva from SF; but splitting the characteristics between three characters leaves you with three underwhelming villains.