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.    

@ebm  I just picked up a pair of Mazda 1960's vintage halo gutter 7308s.  These things are the bomb!  Just glorious sounding tubes.  Much better than my previous Telefunken E88CCs.  I haven't tried the Amperex 6922s yet.  Generally, I have not been a lover of 6922s, much preferring 6SN7 based designs.  But these Mazdas are something special.