As Tony says much will depend on the 'speaker match, but very generally:
For most 80-90dBw conventional direct radiating moving coil systems you'll struggle to beat whatever Cambridge have at the top of the range in the 800 Series. Usually around £1k, very well featured and specified and able to drive most loads. If you need less power just go for a smaller model, or perhaps a Cyrus amp which again always perform flawlessly. Neither will audibly add or subtract anything from the signal. Plenty more but too many to list.
If you are driving vintage 'speakers of high sensitivity these tend to prefer something with a low damping factor. Most valve amplifiers satisfy this requirement well, as do some early solid state designs. Watch for noise though. Valve amps and vintage solid state can be noisy and sensitive 'speakers will just highlight the problem.
If you are driving large panels or designs with multiple bass drivers and want to play loud, this demands current so go for something properly rated for 4 Ohm loads, i.e. something rated to double output into 4 Ohms, and nearly again in 2 Ohms, doing so at RMS with both channels driven - none of this peak power rubbish.
