I can recommend Keithley SMUs in general. I have experience with the 2600 series with respect to solar cell characterization. They are very precise, but the maximum current/power (not pulsed) may be too low for some applications.
I have working experience with 2450. I strongly recommend Keithley 2450 over other SMUs because of its easy configuration in the DC IV. Its having the ability of linear sweep and cyclic voltammetry also.