oof this could be long. For me it largely depends on what it is trying to achieve.
Fabfilter definitely deserve a special mention. I like them because they are clearly designed to be used with a mouse. VERY intuitive, the proportions of what you see on the screen make sense, its very context dependent to the point where you rarely have to customise things. Good use of knob types, colours, and looks similar enough to what we expect an EQ/compressor/limiter to look like.
Valhalla, with the caveat that the newer ones have improved a lot from the older ones. Using Valhalla Vintage or Delay as an example of good, despite the flat look, they use colours, different knob sizes. clever positioning of knobs, a loose left to right and top to bottom design. Vintage was a turning point from UberMod and Room in that all of the controls were visible on the screen at once. So many reverb plugins suffer from having pages of controls to tab through with no priority or even grouping sometimes. Being able to combine relevant controls and prioritise them makes it much nicer and more intuitive to use. They also did well to only show the most essential parameters to the user - careful thought has gone into what the user has access to. Oeksound Soothe uses a lot of the design approach of Valhalla but on a plugin Valhalla probably wouldn't have made. Lots of the points above would be relevant for Soothe too.
I love the Waves Renaissance stuff for similar reasons - they are nice to use with a mouse, everything is visible and logically positioned. Different controls have different knob types (and even different drag directions). Fast, intuitive, no nonsense.
It's not my favourite ever or anything, but for a complex plugin it's a big achievement - Superior Drummer 3, because it's almost like an entire DAW, but it runs nicely inside a DAW without most of the headaches that plugins like this have (say Maschine). It's on the complex end, but it's intuitive, things are where they are supposed to be, it's contextual and organised enough, customisable where it needs to be. Even being able to change the window size and scaling interchangeably of each other is such a nice touch for making it nice to use no matter where or how). For something trying to do so much, it's nice to use.
I really love skeuomorphic designs when done well, but their success is very specific to each particular plugin. as they are relying on the users familiarity with a real piece of equipment. I love the UAD's GUI's for the most part.
With this example, the original hardware already has lots of useful design characteristics that are worth preserving and work well when being used by a mouse. 4 big important main controls that draw your attention? I like being able to click through ratios, I feel like having it the way they do gives some kind of order that you work in (as opposed to if it was on a knob like the other controls). Less important controls either have buttons or different (smaller) knobs and draw less attention. Very few controls in total (just like all the above) but the ones you do have are well considered.
Another random example, but what I like here is it works well in the same way that Valhalla OR the Distressor do. Skeuomorphism has nothing to do with the fact that they used different knob styles and sizes, a left to right direction, a choice of when to use continuous controls and when to limit to less options, evoking similar hardware to give the user some level of expectation and familiarity.
May as well do a NDSP plugin as another type of plugin.
What's good?
- a clear left to right concept for signal flow (input knob on left, output on right, pedals on left, cabs on right, low gain amp on left, high gain on right). Also a top to bottom concept. All very intuitive and easy to find things. Something like Amplitube or ToneX has you moving the mouse all over the screen to perform all kinds of tasks. the NDSP window is using space in a very consistent manner, you're never really going in circles.
- flat controls for utility functions, skeuemorphic for the gear gives a good contrast. On Helix you have Gain and B/M/T having the same visual importance as sag and bias. Our brain has to read the text to know what we're controlling - on an NDSP plugin you can just immediately draw your eye to the first knob and assume it'll be gain. You don't have to scroll though text or click several times to find what you need (think about how many clicks it takes in Amplitude to adjust a mic position).
- Skeuemorphism gives good clues about what gear you are using, how you expect to use it, what sort of sounds it should produce. Pedals use typical colours Green is a tube screamer, orange is a BB Boost. There is good general contrast on the plugin - when something is supposed to pop out and catch your eye, it does, and when something is less important it keeps out of the way. Knobs are big enough and spread out enough so that you aren't accidentally changing settings. You can scroll the amp knobs without your mouse landing on top of a different control - this can be great when using the plugin while standing up, possibly with your weaker hand or reaching at an awkward angle. A cluttered interface requires more precision, more reading, more scrolling and looking around and gets in the way. Readability is always good (and I'd imagine a non native English speaker would have no problems understanding the plugin).
- Only the most essential controls are exposed to the user, and there is a clear sense of purpose with what the user has available on any one page. There is a clear direction from start to finish on what to control and when.
I could do another post of plugins that I just think look nice, with no reference to what they are trying to do or how effectively. Some plugins can look beautiful and not really detriment the use of it, others need to be the complete opposite. So much of a good GUI is tied to what it is trying to achieve.