My opinions (though not on insanity)
Infiltrator: Fun - 8, Viability - 9
-The sniper class shoots my style, and it's fun to use the cloak and act like a ninja. Thinking of making a new one on low level and taking shotgun training. Play it like a vanguard.
Vanguard: Fun - 7, Viability - 8
-I have a good deal of fun with this class, although I die too much for me to have more fun. The fact of the matter is that I tend to be poor at close range classes.
Soldier: Fun - 5, Viability - 9
-I LOVE skills, and the soldier's far too limited in that regard. Its viability comes from the ammo selections, cause a combo of Squad Cryo + any improved ammo is really effective, somehow. Especially if your teammates are equipped with hand-cannons, or shotguns. In terms of versatility, it's second only to the sentinel.
Engineer: Fun - 8, Viability - 7
-I had a lot of fun with this class, for some reason. The best fun you can have as an engineer is using your drone, AI hacking, dominate, and two tech charas/one and Morinth. It's fun to outnumber them with the sheer number of people fighting for you. You can theoretically control 7 people fighting for you, excluding yourself and your party members. 3 from you (Drone, AI Hacking, Dominate), and 2 each from your allies (2 drones, and 2 AI hacking). That's not including if you manage to recast AI hacking/dominate before one of the ones you control dies.
Adept: Fun - 9, Viability - 7
-I challenge the motion that the adept isn't viable on insanity. For one, singularity works on anything, and wide singularity can be comboed into some devastating warp combos. For best results, use Miranda and Samara as party members. Well, not necessarily best results, but the most fun.
Sentinel: Fun - 6, Viability - 8
-I did NOT enjoy this class very much. Yeah, it's viable, but the skills that you use simply aren't entertaining. The sentinel lends itself to be a very defensive class unless you play assault sentinel. If you want to maximize your fun, you MUST go assault.