I'll take a crack at "fastest":
(Note: This should go without saying, but make sure you read through this and understand it before you do anything.)
Assuming your original admin account was "joe" whose home was "/Users/joe"
1. Find User ID Number
ls -nld /Users/joe
drwxr-xr-x+ 65 501 20 2210 May 8 00:19 /Users/joe/
501? That's your user ID number. It may be different, but it probably close to 501.
2. Delete your $HOME
Login as your 2nd admin user, and
sudo find /Users/joe -print -delete
That will delete all of the files under /Users/joe.
There are other ways to do this, of course
sudo srm --simple --verbose --recursive /Users/joe
man srm to see if you want to use
--medium instead of
3. Find Other Files
Then you might want to take a look for other files owned by
joe. You can do this by username or user ID. I recommend ID, but that's probably because I'm an overly cautious fuddyduddy.
sudo find / -user 501 -print
replace '501' with whatever the number was above is Step #1