One is the loneliest number.
If there can be only One, it should be One in a million. One singular sensation.
Most of all, the One that you want.
Let's go with the old, 'If you were stuck on a desert island with only One RPG, what would you want it to be'? Basically, if you could only play One game for the rest of your life, what is that game?
First, GAAHH! ARRGH! The PAIN!
One game?! It's hard for me to picture myself not going bonkers with access to only One game. My personal opinion and outlook on the hobby is that each RPG should support a setting, genre, or style of play and therefore no one game can be the perfect game for every situation.
It's not that I don't believe in the idea of generic or multi-genre systems. I have certainly run Hero System for more than Champions and used systems for subjects they weren't specifically designed for. At the same time, my preference is that the mechanics fit the game and the game the mechanics. I don't like it when my fluff and crunch feel like two separate things that aren't meshing together.
Still, if I only have One RPG at my disposal, shouldn't it be a game that can do a variety of genres and setting decently well?
By my own logic and reasoning, I would choose One game that could do many things or at least most of the many things I would want to use it for. In other words, if I were stuck with but a single system, I would want it to be a system that could run Star Trek, Star Wars, other Spacefaring Sci-Fi, Superheroes, and maybe Giant Robots. So...some variant of the Interlock System by R. Talsorian Games? Mekton perhaps or Teenagers from Outer Space?
The other approach to this is to choose One's favorite game. If I am only able to play One RPG, why not have it be the RPG I love the most. Well friends and fans, that ain't so easy when you're Adam 'Barking Alien' Dickstein.
I love Pork Ramen, especially if it's spicy, but I can't eat it every day. Nor could I eat Tacos every day and damn if they aren't a favorite as well. I'm just not wired that way. I want, nay need, variety.
My favorite RPG at present is, well, ugh, that's another thing that makes this tough. I can't really decide. I am loving the Year Zero system variant I am using for my FRONTIER Sci-Fi campaign, but I am also having a blast with Modiphius Entertainment's Star Trek Adventures. Star Wars D6 by West End Games remains my all time fave rules wise of course, at least for the time being.
In the end, I can't really decide. One of these, One thing will lead to another, and I'll give it One more try.
Oh well. Another One bites the dust.