'guaranteed' is probably too strong a word. Any paypal payment can be retracted upto 30 days after being sent if the buyer sends a complaint to Paypal. The money is then taken straight from your account, and if you don't have the money, you are left with a negative balance and Paypal then jump on YOU to add funds to make it either zero or positive.
Also, don't forget that payments made to a paypal account with a credit or debit card can suffer from chargeback's. This means the money is taken from your account and sent back to the sender, and paypal reserve the right to charge you for them doing so.
The only ways that are almost guaranteed are bank transfers (need to wait a few days and check your bank thought) and nochex (instant, and advertises that they'll shoulder any chargebacks).