Here's how I rank the Trivial Pursuit versions in terms of difficulty. Easiest to hardest.

• Genus III - This version is a joke. The questions are way too easy, and it has too many "Trivial Pursuit" questions. Meaning, TP likes to put in the answers in the questions in a subtle way. I dunno how to explain it. It's like, if a guy named Paul Pusher won the batting title, the question would be like, "This player shoved his way to the top of the AL in batting average in 1998." You know, something like that, a cute phrase that gives a hint to the answer. This version has too many of those. And it has too many TP number questions. It used to be in TP that whenever you had a question like, "How many of every 5 men do this", it was always 1 or 4. And yeah, this version has way too many of these. This is the easiest one.
• 90s - Maybe it's because the 90s are so recent, but this version is really easy. They have a whole category essentially on TV, and it's heavily represented with 90210 and Melrose Place questions. Anyway, yeah, we are the 90s, so it's a really easy version.
• Pop Culture - This may actually be easier than the 90s version, but didn't play long enough to get a sense. But yeah, if it's pop culture, almost by definition it's not obscure, so the questions tend to be easy.
• Millenium Edition - Much better than Genus III, so it's challenging, but it's not ridiculously hard either. This is a good version to play. The only problem is the card holders are completely annoying. They got rid of the old standby holders and replaced it with this completely unuseful card "holder". Annoying.
• Genus IV - This is actually a really good version. They got rid of the number pattern questions, so if they ask a how many of every 5 question, it could be anything. That's refreshing. Plus it's current, so it's challenging because of the questions, not because it's outdated. I'd say in general this is the most fun version to play.
• Star Wars Edition - Unplayable unless you're one of those uber Star Wars nerds, the type that Triumph the Insult Comic Dog ridicules. Utterly obscure Star Wars questions.
• Genus II - The main reason this version is so hard is because it's old. It's just hard to answer questions about relatively obscure sitcoms in the early 80s. It's before our time. So yeah, Arts and Entertainment is by far the hardest category in Genus II (and I), followed by Literature.
• Genus V - Insanely difficult version. Paul Jung actually gave this game to me because it was unplayable for him, the questions are too hard. And he's a bright guy. He's freaking got a PhD. And this version is too hard for him? You need to be Ken Jennings level for this version.
• 20th Anniversary Edition - This is probably, if you removed the time aspect, the hardest version of TP there is. It's seriously difficult. I'd say on average, I know 2 of the answers on each card. For other editions, the average is between 3 and 4, closer to 4. On Genus III, it's closer to 5. But yeah, just really hard questions that have nothing to do with it being old. This just came out. I dunno, it's fun for me to play because it's so challenging, but I dunno how much fun it is for others.
• Genus I - This version is extremely difficult because it's so darn old. The official rules say when you get to the middle the other team gets to choose the color. When we play by those rules, we always choose pink (Arts and Entertainment). Who knows about the arts in the late 70s / early 80s? It's before our time and after Wong's time. It's just impossible. So yeah, just because of those dated questions, this version is really really hard.