I should start by stating that my technical music knowledge is pretty poor. Despite my frequent attempts to learn guitar I've still not progressed beyond three chords strummed very slowly, and have so absolutely no idea about the relative merits of the amps below with regards to sonic quality.
This top five is based mainly on how they look and which bands use them rather than their contribution to the development of electrically amplified contemporary music. Where I have stated specific models this is almost entirely due to the name sounding cool, though in a few instances I did do a bit of research*. (* read Wikipedia)
1. Orange Thunderverb 200 - Hands down the prettiest amp in this list. The Orange Music Electronic Company was founded in 1968 (the same year I was born fact fans) though I only really noticed them on stage fairly recently (late 90s onwards maybe). The cacophanous Jim Jones Revue are one of many recent bands I've seen with Orange amps on stage. As good an endorsement as I can think off. The use of the colour orange is a fine piece of marketing and enables the use of a single Orange amp as a kind of feature wall equivalent in a bank off regular black/grey units.
2. Marshall JCM900 series - Surely there are none more metal than the classic Marshall amp. As an 80s metalhead this was defintely the king of the pile. No self-respecting rock band would want to be seen on stage without a massive wall of Marshalls behind them. The Grateful Dead were famous for playing in front of a vast wall of Marshall's and I once saw Status Quo attempt the same feat (though suspect very few of them were actually turned on).
3. Hughes & Kettner - A recent discovery, of which I know very little. Made in Germany since the mid-80s, I've not yet seen one live on stage. The selling point for me is that cool glass frontage at the top that allows you to see the valves.
4. Vox AC30 - There's something dusty and old school about Vox amps that befits a unit that rose to prominence thanks largely to the British Invasion of the early 60s. The Beatles, The Stones, The Kinks & The Yardbirds were all Vox users. Modern Vox amps have retained that classic styling, but it's that original fawn coloured AC30 that wins out for me.
5. Mesa Boogie Dual Rectifier - Pretty much included because I've always liked the name, it's another amp with Heavy Metal associations. Metallica have been using them since 1986 (the first tour I saw them live), Kurt Cobain used them during the "Nevermind" & "In Utero" tours and Pink Floyd Guitarist, Dave Gilmour, had a Mesa/Boogie head, which he used as a pre-amp for overdrives set up in a send/return chain to the pedal board*, during the wall tour. (*You'll be right if you're thinking that sounds unsually technical, I copied it from the incredibly detail Gilmour repository that is Gilmourish.com)