I voted for a few things, but “Other” was the most important.
I entered the hobby in 1977 with AD&D, followed by Traveller and The Fantasy Trip a couple years later. I added Champions (now called HERO) when it came out. After a period of system stagnation, I eventually wound up playing 100s of different RPG systems, and actually owned over a hundred at my peak.
While the driving forces behind each system I tried varied- usually because someone wanted to try something new- what grabs me about my favorite systems is how well they let me model the PCs in my head as playable characters. With my favorite systems- HERO, D&D 3.5Ed, and M&M 2Ed- that means lots of options allowing for extreme customization. But in others, it has meant that the game mechanics available closely matched the PC builds I wanted to create, even if there weren’t a slew of options.
TL;DR: the main factor is can I use the system to create characters I really want to play.