Because I've owned two and have played through all three of the speakers telemnermonics, (how did you come up with that name?

), mentioned I just thought I'd give my opinion on them.
The closet of the three to what most players think of when they're talking about a Greenback, (in modern day terms), is the 25 watter and it's also my least favorite. It's basically all about crunch. It handles neither cleans nor smooth leads very well. For whatever reason when the '60s ended this became the mold for most future Greenbacks.
The G12H30 isn't really a Greenback at all in tone. Both the 55 and 75 hz handle high gain far better than the 25 watt greenback and also produce wonderful clean tones. Their main downside is they can't produce that woody crunchy sound of the afore mentioned 25 watt Greenbacks.
The 20 watt Greenback, (also sold as the EVH), is easily the best among these three vintage voiced speakers with one single downside. They can produce both excellent crunch and clean tones. Their lead tones are pretty much equal overall to the G12H30 versions with the 20 Watt Greenback having a little more high end than the G12H30 75hz version, more string / note definition as well as being able to cut through the mix better than the G12H30. That said .... they aren't as smooth as the G12H30 when using higher gain and they've always had a reputation for blowing out. I've known of a pair of the 20 watt Greenbacks to blow out when used with a 36 watt Vox style head. The same head that two 15 watt Alnico Blues handled quite well blew out a pair of the 20 watters. I know. Eddie used four in a 4 X 12 Cabinet but his amps were always set up to deliver less output that their power rating through some king of reduced voltage thing and even Eddie was known to keep spare cabs on hand because he was well aware of this flaw. That said .... they were still his favorites and I can understand why. My personal favorites were and are the G12H30s but that's solely because my style requires far less definition and cut through than Eddie's. My tone more closely resembles David Gilmour or Santana so that super smooth high gain tone of the G12H30 works better for me.
If your looking for the finest Celestion lead tones from the '60s that also handled '70s and '80s Hard Rock the best, for most players, that would be the 20 Watt Greenback, (even though not my personal choice). It definitely ranks as #1. It can scream and crunch and speak cleanly. Just don't try to push it's rated wattage. It's not like an Alnico Blue that sounds best when pushed to or passed it's max rating. You can't really trust the 20 watt Greenback to handle it's rated wattage. To me a 4 X12 cab sporting four 20 watt Greenbacks would sound perfect with a 50 watt Plexi or 60 watt Fender Concert head.
If you want smooth like Gilmour or Santana you'll need either the Celestion G12H30 or, even better, Fane Ascension series speakers.
The 25 watt Greenback, in my opinion, was Celestions attempt to build a more robust version of the 20 watt Greenback that, IMO, gave up too much of the 20 watters magic to gain better power handling. I guess not blowing up must have trumped tone because sales of the 25 watt version were such that they became the Greenback standard for long and long. Todays Greenback is still based on this 25 watt model yet most tonal references applied to the "sound of the Greenback" are referencing the 20 watt version. Go figure. Celestion's not going broke. I would be a poor businessman.