1. Look hard at your bike and gear.
2. Make a list of things that can go wrong that you could fix.
3. Make a list of the tools needed to do those tasks.
4. Find a tool or tools that can most efficiently do those tasks.
4a. Determine if anything can easily be replaced to eliminate excess tools (such as replacing Hex head bolts with Allen bolts, so you need only carry one kind of wrench)
5. Shop for a multitool that accomplishes some or most of what you can do for the bike.

Short answer for ME is a Park multitool with a chain breaker a tire lever and a spoke wrench, plus a patch kit, and separate 3, 4 & 5 mm Allen wrenches, a CO2 inflator head and 2 CO2 cartridges (these are for use in case my pump fails, not the opposite) and a Topeak Mtn Morph pump.

Your mileage may and should vary.