How did Drake & Josh's finales (Really Big Shrimp or the Drake & Josh Christmas movie), iCarly finale iGoodbye, and Victorious' finale (either The Slap Fight in production or Victori-Yes in airing order) fare compare to Zoey 101's finale?
depends on what you mean. Z101's always been the "black sheep" of Dan's shows in terms of success and popularity, and its finale was very much overshadowed by the announcement that JL was pregnant, not to mention Chase not being in the fourth season. i think a lot of people didn't watch or assumed it had been cancelled because of the pregnancy, though ratings wise it did decently. in terms of quality, i'd argue it's pretty clearly the worst other than Victorious, but the latter at least has the excuse of being cancelled. Chasing Zoey was always intended to be the finale, so it shouldn't feel like a regular episode or be so underwhelming and unsatisfying. though Dan's shows always tended to lumped in with being cancelled when they weren't, and part of that is they don't have the formal structure to them that Disney Channel tended to have. there was never going to be a wizard competition or a wig off reveal or a graduation for most of them, or some other kind of marker of completion and finality.
in my opinion, Really Big Shrimp is easily the best of them. i never felt the need to watch the Christmas movie (oh, to live in the parallel world where Zoey 101 got that instead) because i was really satisfied with that episode, which was both very funny and wrapped things up nicely, right down to the Amanda Show shrimp joke callback. notably, it is also the only finale that has a co-writer's credit, so for all i know George Doty IV was there keeping a lot of Dan's worst impulses in check. it does what it needed to do, which was put Josh and Mindy back together, show that Drake would be a successful musician, and highlight the bond between Drake and Josh, and then it added in the stuff with the cinema and Helen as a nice bonus.
i've only seen iGoodbye once and i barely remember any of it, other than the really long, painful Dan cameo with the rock collection that manages to make him yelling in Chasing Zoey look quite abstemious, because at least that only went on for about four minutes, and the incredibly heavy handed literal flashbacks instead of just letting the viewer make the connection on their own. I'm curious as to what it would have been like had it not had to build up to the Sam & Cat spin off with the bike, or if the show had been smaller. it was fine overall, though the pacing there wasn't great, and i think the main thing Dan "denied" viewers was a firm answer to the shipping war stuff, which had blown way out of proportion by that point, so i understand the logic there. getting to see Carly's father was also very much a "thing" and i think people at the time liked that the episode leaned into the sentimentality of the ensemble.