Regrettable: Sorry, but it's a tie between three of them. The hand tattoos (The ones on the palms of his hands), the bad Spanish tattoos, and the Tranarchist shit that makes little to no sense, even for an actual transgendered anarchist.
Most infected: It's pretty obvious that the tattoo he got on the inside of his bottom lip is the most infected of them all. He always covers his mouth and his lips are always puffed out. Why else would he be hiding it aside from his horrendous rat/gopher teeth that he hasn't brushed in months?
It's a difficult question to answer since all of his tattoos are retarded scribbles. I'd say his worst tattoo is his infamous Shitty Butt Puerto Rican Bicyclist.
His most retarded tattoo is his pair of handcuffs celebrating spending half an hour in jail.
The thing about the mouth one is that it may long since have faded to illegibility. I used to read body modification sites a lot, and the consensus was that inside-lip tattoos were to be considered just as "temporary" as ones on the palms or soles of the feet, since all three of those areas are covered with tissue that renews itself several times faster than ordinary skin on an arm or ankle. What I read said that if you really want a design on your palm for some reason, you'd better commit to having it redone on a regular basis for the rest of forever, and that inside-lip ones are perfect for the kind of idiots who usually get them (e.g. scene kids who found it hilarious to get "CUM" and an arrow there), because they will look badass (I suppose) for about 30 seconds and then begin their fade into faint-scar territory.
Probably the palm, lip, and armpit ones. The first two since he threw money down a hole for child scribblings, and the latter for the absolute pain it will cause and the rot will be quite hilarious.