I started with Dungeons and Dragons back in the late 70s/early 80s, and I still play it after approximately 35 years.
SLA Industries I started playing in 1993/1994, and still regularly play after 20 years.
Those two games are obvious.
The only game I have actually stopped playing (as in, I turn down invitations to play) has been the World of Darkness setting from White Wolf. After 3rd edition was released, I just did not want to play anymore, it bored me.
But I still play Star Frontiers, Talislanta and a host of other games when I get the chance. Narrowing the selection to just one RPG is an impossibility, and the list of games I have no interest in playing is to long to list.
If you love a game, keep playing it!
If you like a game, play it sometimes.
That is what it boils down to for me, and I have no intention of ever stopping my RPG hobby.
Will have to take a look back on this in 20 years, and see what games I am still playing at that time.