Well I'll do my best to start this off, though I'm sure I'll be missing tons and tons of stuff:
Mark series (I through IV) - the Mark I was pretty much the original hi-gain amp, and I really don't know what was changed in each successive model. I know that lotsa people (John Petrucci, for one) cream themselves over the Mark II C+, though I don't know why; the Mark III I think might be a bit more scooped cuz I seem to remember Metallica really liking those; the Mark IV is the current model and would be the one I'd go for, cuz plenty of people (Lamb of God, Petrucci nowadays) seem to be happy with it rather than going on an ebay rampage trying to find older "vintage" models
Nomad, F-series, Express - Mesa's budget models, in that order; I remember playing a Nomad and fucking hating it, but this was a LONG time ago, when I was a retard at dialing in amps; never tried an F-series or Express, but at NAMM I saw a live show QSC was putting on (greatest live mix I've ever heard, sweet jesus) and the guitarist was using an Express combo that sounded fucking spectacular for high-gain stuff
Rectifier - Introduced in 1992 or so, gargantuan low-end, many think it's a one-trick pony but there are really quite a few options (not the least of which being a switch to go between 6L6's and EL34, Devin Townsend being a user of the latter) - the general consensus seems to be that the older discontinued 2-channel models are better sounding than the modern 3-channels, and I'm pretty certain that almost every tone-shaping option on the 3-channels are available on the 2-channels, it's just that they're organized into more separate entities (like, on the 2-channel, channel one can be either clean or amber, for example). Oh, and the Single Rectifier is two 6L6's, 50 watts, and no tube rectification (so kind of silly to call it a rectifier at all!), Dual is 4 6L6's, 100 watts, two rectifier tubes (which I fucking hated the sound of, ironically, so the single is no loss there!), and Triple is 6 6L6's, 150 watts, three rectifier tubes, and complete overkill. Oh, and on the Dual and Triple, you can choose whether you wanna use tube rectification or diode (and I always used diode when I spent a fair amount of time with a Triple!)
Caliber series - Don't quite know how these fit in; I'm not sure if they were another budget model, but I know a guy who works at my local music store whose opinion I trust loves his DC10 (100 watts, as opposed to the 50-watt DC5), and he's owned Rectifiers, Mark-series, and even an Engl Savage 120
Then there's the Triaxis, which is an absurdly expensive fucking preamp, but pretty killer, as well as the old Quad preamp, which I think was meant to be the preamp section of one of the Mark series
Alright, the ball is rolling, someone else fill in the blanks!