What is your favorite tube type?

I agree implementation matters.  However, a well designed 300B should sound different than a well designed KT88.  

The origin of this questions stems from a brief discussion in terms of the sound profile of 845s vs 300Bs in two well designed SETs and they do sound quite different.  

It got me wondering how many people are loyal to a brand of amp and explore within the brand vs to a tube type and explore cross brands vs. a pure explorer who wants to hear the best and seek the brand that delivers the best implementation for the tube they are listening too that day.  

I thought i would start off inquiring about tube prefernece and see what people’s thoughts are.  

Personally I can't afford to buy a bunch of different amplifiers. Having said that, my amp uses EL34 output tubes and 12AX7 / AT7 small signal tubes. I've used the original Chinese EL34's that came in the amp, the re issue Gold Lion KT77's, 1980's era Tesla, and Holland made EL34's. I take the Holland made over all the others. For the small tubes, it's been the original Chinese tubes, the new Gold Lions, standard Telefunken smooth plate, Telefunken ECC801S and vintage Tesla ECC803S, the latter two being my preference.

This is a hard question, because tubes standing alone don't make any sound unless you drop them on the floor and the glass breaks.  Even if you limit the discussion to optimally implemented power tubes and limit the discussion to only top tier tubes, e.g. Elrog 300Bs vs. NOS Mullard EL-34s,  it is a tough question, because you have to deal with system synergy etc.   But I see you didn't ask what tube was the best, but rather what is my favorite.   So I am going to answer.  6AS7.  It has taken 3 years and a good bit of system and room optimization work to get there, but I will take my OTL 6AS7 amps to my grave.    

However, a well designed 300B should sound different than a well designed KT88.  

Well, triodes do sound better than pentodes, plain and simple- at the same power levels. KT88s are capable of a bit more power than 300bs. Most people run 300bs single-ended while most KT88s are run push-pull. There is a noticeable difference on that account alone.

My preference for power tubes is also the 6AS7G; it allows me to make a triode power amp of high power and low enough output impedance that I don't need an output transformer- making the amp more transparent and wider bandwidth on that account. Due to the fact the amps don't use a transformer, they are more detailed and transparent than SETs (price no object) and I can build them at lower power levels where they work just fine with horns. But the biggest amp I've made makes over 500 watts. Cake and eat it too sort of thing.