Roseanne - While in his mid-20s Whedon became a writer for this show. The episode I know he wrote was a godawful piece of audience pandering slop about Roseanne working at a fried chicken restaurant where her boss, a 17 year old kid (who doesn't look or act like a 17 year old at all) is awful to her. She tries to make a deal with the kid so she can not work weekends and spend more time with her family in exchange for having John Goodman fix the kid's car, but the kid is a dick to her anyway and doesn't give her want he wants so we get a big audience-woo scene where the kid gets tossed out of the house. Gotta be Whedon's low point.
Buffy The Vampire Slayer (movie) - He wrote this. It's so lightweight as entertainment that a gust of wind could kill it, and it's basically terrible--the only good things in it are Paul Reubens doing goofy noises when he dies and Kristy Swanson doing cartwheels in spandex--but it is admittedly sort of fun to watch if you want a trip back to 1991 because you remember what movies looked and sounded like back then.
Alien: Resurrection - He wrote most of it, to my knowledge. I only watched this once, back when I watched all the others. I imagine Covenant is now the worst Alien movie (having never bothered with Alien Vs. Predator) but this was still devoid of inspiration for the most part. Notable for the really really GROSS alien death scene near the end, after it squashes Tuco from Breaking Bad's head. It would be hilarious to me if Alien fans somehow banded together to sign a giant petition urging Ridley Scott to bring back the worthless Winona Ryder character for the new Alien movies he's probably not going to be making after Covenant.
Buffy The Vampire Slayer (show): If you ever plan on watching this I wonder if you'll agree with me that it finally became a mostly-good show AFTER the kids graduated from high school. I've ranted about the things that annoyed me about the high school years before (and the things after) and if you want me to list them for a fourth or fifth time I will. I do believe it was mostly a decent show, but again, more AFTER the kids were out of high school.
Angel - Never watched. Blah blah blah Wesley's a badass on the show blah blah yeah maybe someday I'll get to it. Gimme some time. You don't know who Wesley is. Ask Rich B.
X-Men - "Ever notice how nothing's Whedon's fault? He wrote the retarded frog lightning line, but Halle Berry said it wrong." - Someone insightful on an AV Club comment thread. Some of the sequels to this were better and I frankly never cared for it much.
The Cabin In The Woods - I liked this quite a bit! And he only wrote it, a very funny little parody of horror tropes with an amusing quick ending.
The Avengers - I only thought this was okay. Hey, it had Harry Dean Stanton in it. Tom Hiddleston out-acted all the Avengers and all he had to do was play a high octane version of the usual snotty English villain. Years later we'll all be telling our grandkids about the great Avengers vs. Dark Knight Rises feud of 2012. I didn't see the sequels and kind of don't want to because I'd have to watch all that other MCU crap that's overtaken cinemas to an obnoxious degree.