One kitchen knife to rule them all, or two or three...?

[nereo]

My guess is that a tomato is "best" cut by a knife with serrations, and a large roast by a long meat knife, fillet fish with a thin delicate boning knife, etc.   Or,  you could just chop at everything with a 6" chef's knife and be relatively successful.

When any of my knives can no longer make easy cuts through a ripe tomato without significant downward pressure, that's how I know it's time to sharpen them.  You absolutely don't need serrations.

Agreed.  Serrated knives tear more than cut.  A ripe tomato is a great benchmark for determining whether your knife is sharp or not.  Getting back to the thread subject, a single sharp chef's knife can outclass a drawer full of knives of mediocre sharpness.

[Mikila]

All our best knives are carbon steel.  They maintain a sharper edge than stainless steel.  Three knives are all I need 95% of the time (the other 5% would be be bread-slicing).  If I had to buy new knives tomorrow, I would buy from Opinel.
A paring knife, a small knife for cutting vegetables in-hand, and a large knife for chopping chicken off the bone or large vegetables like butternut squash are my go-to knives.

Paring knife    https://www.opinel-usa.com/kitchen/small-kitchen-knives/paring-knives/set-2-paring-knives-n102-carbon

We sharpen our knives via whetstone and drystone every week or so and slice through all the tomatoes on the first try every time. 

[Goldielocks]

My guess is that a tomato is "best" cut by a knife with serrations, and a large roast by a long meat knife, fillet fish with a thin delicate boning knife, etc.   Or,  you could just chop at everything with a 6" chef's knife and be relatively successful.

When any of my knives can no longer make easy cuts through a ripe tomato without significant downward pressure, that's how I know it's time to sharpen them.  You absolutely don't need serrations.

Oh,  I am not saying you need all these knives,   just taking a stab at what someone else might mean.

[nereo]

Well, I made a DIY knife and it is perfect no need to sharp it, or anything like this. As I am the chef, it is the best knife I had till now.

I have never in my life encountered a knife which truly did not need to be sharpened.  Can you go into some more detail about what you did?