The SH-101 is often my go to mono synth of choice. The sounds are not too complex, the voltage tracks at 1V/octave so it interfaces with my modular stuff well, and the sequencer is deceptively useful (though it only stores 100 notes).
Believe it or not, I actually bought my SH-101 brand new from a music store in 1984. I was in Jr. High and I had won $100 in a contest and it was burning a hole in my pocket. Goodman Music in LA had a deal on the SH-101 where they had over stock and were blowing them out at $160 or so. My parents were kind and encouraging enough to go pick it up for me and loan me the rest of the money (which I ended up paying back through $2/week allowance and helping with my brother's paper route).
My SH-101 has made an appearance on nearly everything I have recorded in some way, shape, or form. Many times, I just use the sequencer to control other synths that don't have a sequencing option, but I also rely on the internal sounds from time to time as well. For deep bass sounds or kick drums, it is perfect and nearly unbeatable. For simple, single oscillator, lead sounds, it sounds great. My favorite thing to do with it is to turn the modulation to noise, then add a little noise modulation to the oscillator to get kind of a melodic static sound - sometimes you can mix this sound in to make a quasi-reverb when layering with a clear, bright, similar sound of the same pitch.
The sequencer, as I stated, is deceptively useful and powerful. Rests and ties can be added and it can be clocked per step. When using it with a drum pattern trigger, the trigger doesn't have to be a steady quarter note, or whatever - it can be syncopated and sending any rhythmic pattern that is programmed or chosen, then the sequencer will follow that pattern. Because it is so easy to program, I tend to use this sequencer for many simple, repetitive patterns. If one wishes to transpose to a different key, then you just hold the transpose button while the sequence is playing back, and press the key that you wish to transpose to. The "C" key is the default, or where no transposing has occurred.