What is your favorite tube type?

With so many tubes that do different things and have different sound profiles, what is your favorite? EL34s wonderful PRaT, KT88/6550s smooth and dimensional sound, lush mids of a 300B, detail of a 2A3 or the huge soundstage of an 845? I am curious which is your favorite. What do you like about it?
A lot depends on how these tubes are used- and the output transformer (or no transformer at all) as well!
It all depends on what speaker that I am using. I like the bigger brighter sound of the KT88 with my current speakers!!
I love EL34’s. Smooth Highs, Engaging Mids and Rich Bass. Currently using Russian Tung Sol EL34B in my LTA ZOTL40 Power Amp.
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!
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.
I agree with Ralph that implementation outweighs the actual tube. That said, I'm quite pleased with my current NOS Mullard EL34's and slapped some into my guitar amp too. I've always wanted to hear some GM20's.
I'm on my second set of Gold Lion KT88's - running in my PrimaLuna Integrated. I've tried the EL34's, KT120's and 6550's. But the 88's are my fav.
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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.
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? 
@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. 
My two favorites right now are Sofia 6SN7(lush but detailed) and Tung Sol KT150(powerful and airy).
Depends on where the tube sits. My favorite tube of all was a black plate CV5042 (of uncertain heritage) which is a military version of the 12BH7A but it was better than those. It had silent blacks and huge soundstage with exceptional detail in a cathode follower circuit.
I also loved my quad of Marconi-Osram GEC KT88s in a Tim deParavicini power amp. Delicate, tonally colorful midrange and sweet highs. Bass not great but...
Currently, my favorites are the @wolf_garcia mentioned TungSol 7581A which are seamlessly tight and clear and powerful in place of 6L6 and EL34/KT77 and, the PSVANE Globe 6SN7 with its unbelievable detail. It needs a lot of burn 
My favorite=a pair of single ended sv83's series straped to mono on woofers and single ended el34 on tweeters. Both putting out about 5 watts.