What is your least favorite architectural style and why don't you like the style?

My least favorite style is Post Modernism for some particular reason this style is not warm and welcoming to me as a cozy craftsman and lacks the ornamentation on a Victorian! To me they look bland!

I can't stand Brutalist architecture- to me, if it looks like a missile bunker, the only thing it should be is a missile bunker. And of course, the building that housed my major at university came from that time period, so it felt like I should still be looking around for fallout shelter signs and reporting my neighbor's suspicious activities to Joe McCarthy... plus, the building was falling off its foundation. I personally enjoy the post modern and ultramodern styles that are popping up lately, but I have a feeling that those are the buildings that'll end up looking horribly dated in another couple decades. I wish there were more classic styles being built. Rustic architecture, Colonial, Georgian, etc... especially on college campuses. Academia needs white columns and red brick!

Probably one of my least favourite styles has to be the Neoeclectic/McMansion style used in American suburbs since the late 20th century. *shudders* It brings me out in a rash just thinking about them. There are only a couple of exceptions to my distaste of that style, and those are the ones that could so very nearly pass as horribly updated historic homes. At least Postmodernist and Brutalist style architecture is trying to make some kind of statement. Neoeclectic is just blergh.

I'm with Fergie on the McMansions. I find them too heavy-handed and without any grace.

I'm also not a big fan of mid-century stuff. Mostly, I remember hating the colours of the houses. (Base housing was almost all mid-century when I was growing up.)
And those bathroom colours! Pink and turquoise tubs and sinks? Harvest gold and Avocado Green stoves and fridges? Oy. Not for me. I like the proportions of mid-century, and I can appreciate the sculpture of them...but...eh, just not my cuppa tea.

I don't know about disliking any particular styles of architecture. It's like asking what types of food I don't like, because while there are so many individual examples I don't like, almost all types of food have a few individual examples I do like.

Three architectural design elements I definitely don't like are form without function, excessive ornamentation (or pretty much any ornamentation that distracts from the overall design of a house, deliberate or not) and flimsy-looking houses.

Form without function:

Self-explanatory. That metal arch thingie serves no purpose other than to say, "Hey, look at me, I'm...something."

Excessive ornamentation pretty much includes most of the renaissance styles. Here's an example someone posted in another thread:

I'm sorry, but that's just an ugly rectangular box with a lot of ornamentation, which in the end serves no function. I know, to each his own, but since the thread asks what I don't like, well...

Flimsy-looking houses can be summed up in one word that epitomizes them for me - Neutra. As in, Richard Neutra.
There are many examples on Google Images with a search for Neutra architecture.
What many if not most of his houses, especially the more famous ones, have in common are big, flat roofs and cantilevers that are held up by thin, seemingly flimsy columns. The columns may be made of steel and not actually be flimsy, but they look it. In other words, the proportion is wrong, at least to me. Many of these houses look like they would crumple under the load of a heavy wind, which they haven't of course, but they look it.
Sorry, Nina, but this also includes Mies' Farnsworth House.

I myself always felt that the 1960s and 70s 'Brutalism' style was the most poor attempt at 'modernism' which destroyed British towns and cities on a large scale but fortunately the style has not been repeated

Granted, there are many examples of large and ugly brutalist buildings, most of which seemed to be municipal or corporate buildings of some sort, or large apartment complexes.
However, I would submit that most of the private houses done in this style are quite beautiful. Here's a Google Image search for brutalist house, which shows many private homes that are beautiful, imo.
I love the brutalist style when it is used on residental homes. Brutalism forces the architect to create beautiful shapes and spaces without relying on the various crutches of ornamentation, colors and other distractions that generally don't serve any real function.
A little ornamentation can be used merely to highlight the larger design aspects, and to provide contrast without becoming a focal point itself.
(Am I sounding sophisticated enough, like I know what I'm talking about? 50+ years of bullshitting does have an effect, after all. )
Luciano Kruk is an Argentine architect in the brutalist style that I like.

I agree, when done right, the style is not too bad and can work well in landscaped settings.

The worst examples tend to be housing estates which were build quickly and cheaply and suffered decay due to neglect which put the style out of favour

Nice Sims house as well