The Euro.
It’s like play money. I buy something, then an hour later, “Damn, did I pay $40 for that?”
I like ones, fives and tens. I go to the farmers market very early most Saturday mornings and they appreciate me having correct change while they’re still trying to get set up. And I make the day of the cashier at our work convenience store on Thursday afternoon when I buy a five cent piece of candy, hand him/her a couple of 20’s and say to give me as many ones as you want. On the other hand, when I stop for a breakfast sandwich 10 minutes after the restaurant opens in the morning I use a 20 and get three fives in change for the same purpose. Per #9, they probably hate me.
Stuff I actually use: the quarter.
Stuff I would like to hold in my hand just to say I had done it: the 2008 Zimbabwean $100 trillion note.
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I love the $100 bill. I play poker, and if I have a walletful of hundreds, it’s because I did well. Hundreds means I played good poker.
Not everyone takes them but I deposit them in the bank anyway, unless I’m planning on a trip to Costco or the grocery store anyway, as they both take them.
Those 100 kg Canadian gold coins are OK, if a bit too big for most pockets.
Who uses cash anymore? This isn’t 1985. Over 99% of my purchases are with my United Airlines credit card. I fly to Hawaii free a few times a year.
Dimes. They’re shiny and have nice hand-feel.
The Museum of the Bank of England (London)has a very nice display of old currency. It’s free to enter.
My all-time favourite banknote: the 1982 Netherlands 50-gulden note. What can I say? I like yellow. 
Other faves:
The Canadian $20 note with the Haida art. The current polymer $20 is pretty good. The Canada 150 commemorative $10 is really good. And I am anticipating the upcoming Viola Desmond $10.
It has a cool name, the perfect size and weight, and a pile of them make a pleasing sound clanging together.
I’m partial to the Scottish £100 note. Mainly because a mate kept insisting they didn’t exist, so when we ate out together at a higher-end restaurant and the time came to pay, I duly pulled one out… 
I’ve taken to keeping one as an emergency reserve separate from my wallet. So when my nephew was given one for Christmas a while back he too was suspicious, so I told him to compare it with a known real one…
Are we including hypothetical currencies? Because if so, I just have to throw in a hat tip to my favorite hypothetical currency of all time, the Dead Child.
According to Population Services International, a respected charity research group, it costs between $650 and $1000 to save one child’s life through charity. You’ve probably heard lower numbers like twenty cents somewhere. The lower numbers are wrong. Yes, maybe an anti-measles vaccine for a kid in Africa only costs twenty cents, and measles can be fatal. But there’s a lot of overhead, and you have to immunize a lot of people before you get the one kid otherwise destined to die of measles. I find the $650-$1000 figure much more believable. Let’s round it off to $800. So one dead child = eight hundred dollars. If you spend eight hundred dollars on a laptop, that’s one African kid who died because you didn’t give it to charity. Distasteful but true. Now that we know that, we can get down to the details of designing the currency itself. It should be a big gold coin, with a picture of a smiling Burmese child on the front, and a tombstone on the back. The abbreviation can be DC.
[…]
Yes, you grudgingly admit, such a system is technically feasible, but why in blue blazes would we want to replace our reassuring green dollar bills graced with dignified ex-presidents with that? I leave that question to an article I read on the BBC site today: woman spends £250,000 on a luxury doghouse for her Great Danes complete with spa and plasma TV. This does sound sort of ridiculous, but clearly it is not ridiculous enough. After all, at least one person thought it would be a good idea. Clearly, saying “doghouse that costs 250,000 pounds” does not carry the appropriate punch of “do not buy this.”
And that’s why I recommend switching to a dead-child-based currency. “Doghouse that costs 250,000 pounds” might not carry the proper punch. “Doghouse that costs 500 dead children” does. Using dead children as a unit of currency carries a built-in awareness of opportunity costs. Yes, you can buy that doghouse, if you really think it’s more important than spending that same money to save five hundred Haitian kids’ lives. Go on! Dogs watching plasma TV! That sounds adorable!
As far as actual currencies go, I love the 1€ and 2€ coins. I hate that, when I’m in the US, a pocket full of change is rarely worth a slice of pizza. In Germany, a pocketful of change can buy me a beer!
My favorite US currency is the $5. Just, Abe.
I like the look of Euros and Hong Kong Dollars.
My favorite currency is Vietnamese. The dong. Heh.
I have some older beautiful dongs with a tiger on them.