Some good news, for a change . . .
If you find yourself in the fortunate/unfortunate position as sole beneficiary of a significant inheritance, I'll spare you my trial & error blind alleys, roadblocks, and misfires, and instead share with you what I've found to have worked well, thus far:
1) Set up as many of your accounts as possible to be "joint tenancy" accounts. I did this in only two instances, but it made life very easy. In hindsight, I wish I had made all the accounts "joint" . . .
2) Second easiest is to establish your own account at the same institution as your relative -- be it bank, credit union, or brokerage. When a POD (for banks) or TOD (for brokerages) situation comes through, you can simply accompany the death certificate (a copy often suffices, sometimes by email) with a letter of instruction advising your FI to transfer funds/holdings from the deceased's account to yours.
3) Current interest rates may not be to your liking at existing accounts. So when you want to move your inherited funds around it is important to find a combination of institutions to work with that have the least friction with each other. My holy trinity is: any Shared Branch (for credit unions); Chase Bank (for overnight processing of certified "bank" checks); and online United Bank (Connecticut) for same day transfers to their high interest bearing Money Market account (mine currently at 2.5%). All these institutions have seemingly unlimited capacity for dealing with large sums of money, without any picayune caps.
And though ACH withdrawals from United are limited to six per month (per Regulation D) you can write as many checks as you want, for as much as you want, without concern for Regulation D.
As an example, I went to my local credit union yesterday, pulled available balances from two Shared Branch credit unions, deposited a portion of the total into a third credit union, and left with two checks. I deposited these checks into Chase, which instantly credited me with $200, putting the rest on hold, But the very next day the entire deposited amount became available. And so from United Bank's website I scheduled a pull today, which has already posted there (and is presumably earning interest). Not bad -- considering these were all six-figure transactions.
Again, I'm sparing you the horror tales of what did not work: mostly banks -- both online and brick & mortar -- with annoying caps that make it difficult for you to take care of business, interfering with the smooth completion of intended transactions (closing accounts; opening others) either through delays, or limitations on daily/monthly transfers.
I'm happily sharing what I've found to be most beneficial for beneficiaries. And I'm also curious to hear from others what kind of "hub" accounts have worked best for them. These days I'm on the lookout for places to safely and easily stash cash, staying under the FDICs $250k insured amount. So bring 'em on -- the more, the merrier!