In the past it was tyres. Used to agonise over the perfect tyre for a particular bike. Where and in what conditions I was going to ride it. When I started running tubeless, changing tyres became a right faff to do often and I just fitted a tyre and rode it until it wore down. Modern tyres generally have a much wider sweet spot than in the past and as long as you choose roughly the style, it’s hard to go wrong. Have to say, I’m much more content now.
Well, except when it comes to grips. Still a sucker for every new wonder grip they come out with.