What is your favorite tube type?

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.

6v6g st in a 4 watt amp

My recent faves after trying various types have been the new Tung Sol 7581a, but I recently bought a Dennis Had "hot rod" SEP "high output" (12wpc or something depending on tubes) that's around 6 months newer than my current Had "fire bottle," and it seems to sound best with new Gold Lion KT77s. The "hot rod" has more voltage according to Had, and the internals are nearly identical to the "fire bottle" except for an extra Jensen cap or two...the two amps are VERY similar sounding and I'm not sure yet which one sounds better, and I'm having trouble locating any info on what, exactly, is the technical difference between these two amps...anybody know the answer? 

Love Amperex 6922 or 6dj8 and 12ax7 as well sounds wonderful!!