Zucchini seems to be the most popular veggie (or fruit) in my tanks. I take a slice and weigh it down with a
screwcumber This link allows you to get it at their regular price, but part of the payment goes to a donation to another great website called planet catfish. Some folks peel their zucchini, but I don't. That's the favorite part for my plecos. Mine also like sweet potatoe, summer or yellow squash, even carrots.

It usually only lasts about a day. I also have malaysian trumpet snails in those tanks, and they take care of any leftovers.
P.S. A lot of folks also feed cucumber, and I have in the past also. I quit feeding it though because the seeds mess up your tank, and I've read the peel is actually poisonous for humans, but we eat a much smaller amount in comparison to our body weight than our fishie friends do, so I don't think that's a good thing. There's so many other veggies out there.