Definitely alone. My friends are usually not on time when I'm free during gaming... Though a lot of online multiplayer games (at least the games I play - OW2 and Call of Duty) are alone versus the world.
I've been having more fun recently - I'm luckily good enough to guarantee wins for the most part. Even though the game itself for the most part isn't as good as CoD or anything like that. Free suits me well.
I've got literally hundreds of single player games on Steam... and hardly every get round to playing them lol.
I also play online with my friends off and on, depending on how often the group plays. Games are either Rainbow Six | Siege (most commonly), or the latest CoD.
I have to admit that the human factor really makes a difference here and even though I've got a gaming PC, I bought a PS4 specifically to play with them a few years ago, which has been a great investment.
Depends on the game. Most games where I can craft, build, and take my time with getting quests done and looting everything.. I like to play alone because I enjoy the freedom of doing things my way. But for multiplayer games, I do enjoy playing with friends.
I'm like @Uncrowned....depends on the game. If I'm building a room or a fortress, then solitary is good because it's restful. MOBA is different by it's nature, and interactive necessitating social interaction.