What is your favorite tube type?

Best PP low watt power tube: 6BQ5/EL84! A pair of these (15 watts) with an Altec Valencia or an Altec 19 will get you halfway to audiophile heaven! Used as a quartet in the famous Vox AC-30 guitar amp!

The ne plus ultra power tube: 6LF6 used as a PP sextet in the Julius Futterman hand-made H3aa OTL mono amps! I have a pair!

6BQ5

I've run a lot of different KT88 and 6550 in two different amps.
They are all different but honestly not by a lot. Not enough to be worth changing tubes just for the sake of changing tubes the way a lot of guys do. That said, my favorite for its deep stage, liquid ease and lack of glare is the JJ KT88. The JJ is also one of the least expensive of all the KT88 and 6550 tubes I've tried. 

Currently running the Tube Store Preffered Series KT88. One failed dramatically within a few weeks, and they are all noisy and not nearly as refined as the JJ.

I run 300b's currently and do like what they give in the midrange I like the Gold Lion 300b best ( I will never be able to afford NOS 300b's). But I would say 6550's are one of my fav's they just seem to get the music right. I do like the NOS 6550 best though I found many of the modern ones a little dry sounding but it has been a few years since I had a amp that used them.