Worst issue in modern football kits is all the retro ******** going on. Not every kit from the early 90s was "iconic" many were genuinely dreadful. Also where is the creatibity in modern football kits? Nobody takes risks with design any more. Much like modern filmmaking and rehasing tired films from the 80s, who the **** wants to see the new Indiana Jones or some crap remake of Ghostbusters? Make new things!
I actually cant think of a single modern kit that was "out there" and creative with design that will be remembered fondly. Meaning in 20 years time we'll still be talking about kits from the 90s most likely. Sad reflection of modern society's malaise generally if you think about it.
Overall the worst football kit I have seen is probably something from Liverpool, there have been some shockers in the New Balance era. Puma made some dreaful kits for Arsenal too, last season away was **** as well.
In terms of the best kits, I recall a Leeds kit from the mid 90s that was half blue and half yellow that I remember was very pleasing. Spurs have had some nice ones and the white/navy is actually very smart generally.