Class 3/4 is plenty fun and perfectly safe. There are people out there in there 60's who have no trouble with it. The most important thing to remember is if you go overboard, don't try to stand up.
k I guess if you like wearing a skirt :^D

Class 5 is pretty safe in a pig boat. A lot of it depends on the river. It can be a low consequence difficult run, or a high consequence easy run. A reputable company won't take a n00b down a dangerous river if they don't have experience.
Better than a raft is getting your own hard boat. I like open canoe, but a kayak is ok I guess if you like wearing a skirt :^D
But, a canoe isn't going to be very successful above roughly class 3. Though, I've seen some class 2 that'll submerge a canoe quickly, and I've seen class 3 that a canoe would glide right through with no problems. If you're forced through a standing wave (rather than over), and there's no way around, you better have a skirt.
What's the most exciting class without having to die.
Friends want to go to kern river. Not sure if we are bringing the women, but most likely
You're not at a risk to die on any of the major class 4/5 rivers in the USA that are regularly run by outfitters. You'll probably be bored on Class 3, so look for a decent Class 4 in your location and go from there.
You can check youtube for vids from the popular rafting rivers to get an idea of what to expect. The one thing I'd suggest is to research the rivers for the quantity of the action, not the maximum class of the toughest set of rapids. A river that is all class 3 and 4 will be WAY more exciting and more fun than a river that is class 5 in a couple of spots with long stretches of flat water in between the rapids.