A savings account of some description at the same bank as your checking account. Transfers between accounts owned by the same person at the same bank are instantaneous from my experience.
I always keep a balance in my checking to ease my mind, but the bulk of my monies I keep in a savings account next to it and I transfer as needed prior to paying the bills.
I bank at Alliant Credit Union, and there's no restriction, ever since Reg D took effect. It's a pretty solid credit union, which almost anyone can join, if folks are interested in a commercial bank alternative, but aren't all-in on brokerage.
I sum up a reasonable estimate of what my monthly expenses will be (with a small safety margin) and move that from my savings to my checking before monies need to start moving out. That's one transfer. If I find myself short, I make additional transfers as needed which isn't often.
If I screw up (shit happens) I have overdraft protection so the checking just pulls from my savings with no overdraft fees, but ideally this never happens.
Of course, none of this actually matters because transfer restrictions on savings accounts were lifted as pointed out by a sibling commenter. The only reason I don't just pay straight out of my savings is because I simply prefer to keep outgoing monies to my checking account.
I always keep a balance in my checking to ease my mind, but the bulk of my monies I keep in a savings account next to it and I transfer as needed prior to paying the bills.