I've tried 4-5 different kinds..TS 6550s, Sovtek 6550s, EH 6550s, Svetlana 6550s, etc.
Svetlana 6550s are the best for me honestly. I have them in a Mod100, Mod50, and a RT2/50, and LOVE it in every amp.
I've been running 6550s in amps for 6-7 years...Marshall 9200, VHT 2/50/2, VHT 2/90/2, VHT 2150, Marshall JCM800, Marshall Silver Jubilee, etc. and it's by far my favorite tube.
I did a test on my VHT 2/90/2 putting KT88s vs Svet 6550s. For me...the KT88s lacked the sweet, juicy, thick mids that the Svet 6550s have. The KT88s packed a punch...but in a mix the Svets just sounded better, less harsh high end, and just have the tone I wanted. I feel the Svet 6550s really lend themselves tonally VERY WELL across the modular spectrum. I find them a very underated power tube (just like the Celestion K100 is underated for its versatility as a speaker!!).
I can get killer clean tones w/ it, killer JTM45 tones with it, killer marshall tone with it, etc.
It's by far the "island power tube" for me.