Fortunately, there aren't nearly as many unusual effects as there are hats, so I'm happy to list them out here - though I might miss a few! Please note that this is all subjective, and I'm sure that some people will disagree with me.
Includes effects like Burning Flames, Scorching Flames, and Sunbeams. Also includes some exclusive Halloween effects like Cloudy Moon and It's A Secret To Everybody. These are the best and most expensive effects that you can get, and they look absolutely <em>fabulous</em> - and, as you'd expect, they command a premium price. Hats with these effects can cost anywhere from 4 buds (cancer hats) to 100 buds (the Burning Flames Team Captain is an example of this).
These effects fall just a little short of being the best available - this tier includes Green Energy, Purple Energy, Circling Heart, Haunted Ghosts, and Cauldron Bubbles.
Now we're at the effects which not everyone loves, or which are valued lower than the rest. In this tier, we've got effects like Vivid Plasma, Searing Plasma, Stormy Storm, Blizzardy Storm, Smoking, Steaming, Orbiting Fire, Eerie Orbiting Fire, and Orbiting Planets. These effects aren't the worst by far, but they're not the most desired either.
Now we're in the realm of "oh my gosh, this hat is going to be hard to sell unless it's a high-tier hat". The effects on these hats are either ugly as heck or aren't visible enough for people to notice in the heat of battle - this includes stinkers like Orbiting TF Logo, Orbiting Peace Sign, Massed Flies (occasionally worth quite a bit with certain hats), Bubbling, Green Confetti, Nuts n' Bolts, and Purple Confetti. The absolute worst hats with these effects will sell for as little as eleven or twelve keys.
(Purple Confetti Furious Fukaamigasa. $24.)