Puzzle or skill/twitch are probably easier.
Enemy AI and physics, two of the most difficult parts of coding, are effectively optional for both genres.
Character models/sprites are optional as well, meaning less time can be spent on art and more on programming, which is an important conservation when you're working alone.
Think Tetris or
Sinuous, both of which were critically acclaimed and I believe were made by one person each.
However, I feel like easy isn't what you should be going for. Fun is. You'll be working for a long time with no fuel other than the entertainment you're getting from making the game, and the desire to see the finished product. Coding isn't always fun. It gets boring and stressful quite often, actually. What keeps you going is how fun you know the game will be when it's done.