What's the best multitool? Depends on what problems are likely and your access to help.

I do day rides of up to 75 miles from home. If I get truly stuck I can call home for a ride, or call a cab, but I have never had to. Here's my multitool:
It has 3mm, 4mm, 5mm, 6mm, and 8mm hex wrenches, and a small flat-blade screwdriver. For the riding I do, this is all I need.

I have never had a chain problem in over 40 years of riding, and on a three-month cross-country tour, so I don't carry a chain tool (I did on the tour).

My most frequent on-road problem is tire punctures, typically one or two per season. I carry an extra tube, patch kit and a pump. I don't carry tire levers because I can remove and install my tires by hand.

My next most common problem is, uh…hmmm…

A few years ago a derailer cable broke. I used the flat-blade to adjust the low-limit screw to a middle gear and rode home. My fault for lack of maintenance on the cable.

That's it. Some of these jackknife-style tools look capable of totally rebuilding a bike, and if I were going on a tour, I'd carry one. But I maintain my bike at home, and I'm not going to pull a crank or true a wheel on the roadside.

Off-road riders have different problems and may need extensive tools, but for a day trippin' road guy, this little Park tool is all I am likely to need. (Famous last words )