You forgot the diminutive but highly sought out EL84!
Here are some generic thoughts towards these tubes:
6V6 & 6L6: Close cousins. The 6V6 is about 60% of a 6L6GA in regards to power. They hold similar properties in regards to efficiency, late breakup and a clean, natural sound.
EL34: Earlier breakup in most cases. A rugged, inexpensive tube. Less efficient than a 6L6, but in the right configuration- it's a touch warm and good at driving complex speaker loads. Thanks to a large plate structure and slightly different internal makeup than a 6L6, more power can be produced and they will generally accept higher plate voltages.
KT90: A large EL34 in my opinion. (The Ei version reminded me of an EL34 and F2A combined!) Too bad it's not around any more. Other brands may be different and most likely are.
KT77 & KT66: Never used them, so I can't comment.
El84: Stands on its own. Powerful for the size and extremely configurable. I have found these tubes to be VERY easy to work with. An excellent choice for the DIY crowd. They tend to be 'musical' and not quite as clean cut as a 6L6/6V6. It doesn't detract at all from the presentation and adds depth to the sound stage. El84 sound is unique and can be breathtaking in a proper installation.
My favorites? All of them.
