I've been a solo founder at 5 out of 6 startups. I'd highly recommend having a co-founder (if you find the right one).
Starting a company has extreme ups and downs. It's an emotional roller coaster ride. Having a co-founder makes the burden easier to carry, especially in the early days.
The key is having a co-founder who is great at things that you're not so good at.
So, if you're a really good engineer, but design is not your forté, a co-founder who is great at design & UX will be far better than another technical one.