Not a fan of multitools. I find they are rarely as good as specific tools and usually have stuff that particular bike doesn't need. I like to stay fairly minimalist but I bring all the Allen wrenches that bike needs (except 8mms for cranks). 2 tubes, patch kit, two tire irons. Some bikes get spoke wrenches. (Needed to nurse heavily dished rears to ridable after right-side spoke breakages but my fix gears will limp home just fine minus any one spoke doing nothing.) Wrench for fix gear rear hubs. I should be better about carrying spare links and a breaker but (knock on wood or keep riding the fix gears) I've done OK so far.

On "rides" (as opposed to commutes and city rides) I always bring a Leatherman on my keys. The simple Boy Scout/woodsman philosophy. They are really good tools and very versatile. If the completely unforeseen happens, that is the tool I'd request if I was told I had to pick one tool 10 hours before I was told what the situation was going to be. (It has shaped wood to make "get me home" fixes a few times. Bent metal (those pliers are really good; they get used at home where they sometimes serve better than any of my good pliers. And the tiny screwdriver is perfect for undoing zip ties so you can use them again.) Still looks and feels like new. My sis gave it to me in the '80s.

Edit: All my bikes get Zephal pumps. And now both presta to schraeder and schraeder to presta adopters. (The latter primarily to help others without messing with the pump head and maybe losing parts. Cantitoe Road carries them for $9 and ships promptly.)

Second edit: Boots - I always leave with enough bills to have 5 or more in my wallet after any stops for espresso or the like. US bills are high quality, strong, light, very versatile and reusable after getting you home. (Might have to be laundered. Shhhh.)