This is a tricky but good question that I find a lot of people actually get wrong. You see there is no amp maker that is truly superior to another, but rather different amp makers cater for different styles of music and so can achieve different tones. Always listen to an amp very critically before you buy it.
Here's my low-down on amp-makers that I've used in the past:
Marshall: very good for a classic rock/old school metal sound in the valve range, and are currently used by countless professionals, however the transistors are a way behind other makers in quality to me.
Fender: the top amp maker for those wanting to get a clean sound, but very little gain quality, even when pedals are used to compensate.
Vox: very good classic-rock style amp maker producing nice tones for stuff like Bad Company, but other makers may retain more clarity of tone.
Blackstar: the new boys but with fantastic tone flexibility due to the ISF (infinite shape feature) installed, great for almost all rock styles, and have surpisingly good power, but are comparitively expensive.
Line 6: known as a metal amp maker, they have a ridiculous amount of built inn features for getting tones, however they can sometimes fail to deliver a specific tone as a result.
Bogner: Mr Bogner also designs a lot of the Line 6 valve range, but his own amps are far better. Fantastic reviews are given and I use his Alchemist stack presently. However he is a boutique amp make so they are sometimes pricey, but head to head would destroy any marshall of similar price. The least powerful amp he makes is louder at highest volume than a kerry king signature marshall.