Best:
Dictating your own routine that doesn't have to accommodate other people, basically freedom to do what you want when you want
Likewise, someone else's routine won't disturb you, in my case this is my partner watching YouTube etc when I'm trying to sleep!
Decorating and furnishing your place however you bloody well like (obviously within budget and still within rent agreement when renting)
Lots of space in your bed!
Not having other people's shit being in the way/taking up space
Peacefulness, basically not having to tolerate shitty or abusive behaviour in your own home
Don't have to wear headphones! (Obviously keep neighbours in mind)
Generally only have to cook for yourself, clean up after yourself, etc.
Have full control of household budget and finances, don't have to worry about accommodating others in this.
Can get a cheaper/smaller place to rent or buy, don't need x number of bedrooms
I could go on really... I loved living alone!
Worst:
Can be a lot more expensive... (although I was better off financially living alone, so depends)
Can be lonely of course
Cooking for one can feel like a faff!
Can slip into neglecting yourself as don't have someone else around to keep you in check (some people thrive better alone though)
If you lose work etc, don't have another income to keep things afloat