Times New Roman is very plain and “old-fashioned”, so avoid that one.
Arial and Verdana, as already mentioned, are good ones, but my particular favourite is Calibri. It’s plain and simple but looks nice and is pleasant to read.
In short, avoid fancy fonts such as Vivaldi, funky ones such as Jokerman and tacky-looking ones like Comic Sans. Even for your headers/addresses, don’t change the font to something jazzy that you think will stand out.
Another thing – use the same font all the way through. Don’t have a different font for the address or the closing or each paragraph (whatever weird and wonderful things you plan to do!).