DMD: The Shadow
Alpha numeric: TX-Sector and Banzai Run
Numeric: Black Hole
Electromechanical: Four Million BC
I've owned these for quite a while. The Shadow for 15+ years. For different reasons, they all withstand the test of time with flying colors. 4MBC is the only one of the bunch that I had to let go at some point, and I still miss it.
But, as others have said: variation is definitely essential to keep things fresh. Which is the main reason why I don't stick to one manufacturer or time frame for the games in my collection.