That's a good question. I'm fighting off saying the old ones!
The valvetronix type ones, anything with modelling, may be good amps but they don't give that "my fingertips are physically linked to the sound coming out of the amp" sensation. If you don't use pedals much the straightforward valve one with treble boost is amazing. I believe (I've been told by good people) the latest ones are good but I haven't tried them. So I suppose the real answer is you'll know it if you try one because you can feel it. It's so personal isn't it...And also,what works at home doesn't always work for a gig so that needs taking into account too.
Just to negate all I've said so far...... I've fitted with a simple Award Session transistor amp perfectly successfully, I don't think the audience can tell the difference, and that's slot lighter and more reliable. But a really fruity amp ups my performance I believe....But it's only a belief. One meets those guitarists who can make any amp sound amazing... Don't you just hate them?
Nick