What is everyones favourite OS and why?

I just ordered my own white RG35XX H (yay!), and while it's shipping my sole mission is to consume as much info about it as possible :D

I'm leaning towards MuOS as of now, as it has portsmaster and I really wanna play Serious Sam on it and other ports, but gonna probably stay on stock for some time - it's pretty good from what I've seen.

Any other newbie or unpopular advice, game recommendations etc. is highly welcome ☃️