I have less experience than some of these guys, but I've been playing for about a year, and I think I have some input that may be useful.
First, on devastators: I used to love these guys, too, until I realized that, more often than not, they are overkill. This is not because I'm hugely lucky (I'm not)--you just don't see many lists with lots of land raiders or Leman Russes (Necron lists with lots of monoliths being the exception). A single twin-linked lascannon shot from a Predator Annihilator will almost always hit, and usually damage something with AV 12 or below. However, while this is true, the far more valid complaint about devastators is that they are very, very static. A decent opponent (or just one without a lot of armor) will be perfectly capable of keeping his vehicles out of your LOS, unless you have a lot of devs (impractical because they're so expensive) or are really good at deploying them. Mobility in your AT is really important, I think.
I personally don't use las-plas squads, so I don't have much input on them. They don't really fit with my list, which is very mechanized and, if not terribly assault-oriented, still relies on getting close. While way cheaper, they suffer the same weakness as devs in that they're too static.
Dreadnoughts I like quite a lot, though not in the way they've been talked about here. I take a venerable, tank-hunting dread with twin-linked lascannon and a missile launcher. I fully understand that it's crazily expensive for an AV 12 vehicle, but the anti-armor devastation it brings is more than worth it. That's a twin-linked s10 shot (essentially--I know it's not actually s10) and a s9 shot, both of which can fire while moving. Furthermore, the dread doesn't get immobilized in normal terrain--so you can start it behind area terrain, and then march it up, guaranteeing a few shots. The venerable upgrade, by itself, isn't bad either--it makes it surprisingly more survivable, especially in cover that makes it obscured. Finally, it's perfectly capable of tying up a unit in CC, as long as they don't have any fists or things of the like--such as Dark Eldar Wytches, or other units like them--it doesn't devastate things like a normal dread, but it can simply keep a unit from being scoring, or allow you to counter-charge.
Powerfists are, in my mind, absolutely essential. You know that dreadnought thing I described, with a ranged dread tying up a squad indefinitely? It can happen you you, very easily. A dread simply will not charge a unit of marines with a fist, because it will die. It also makes your squads effective against things like monstrous creatures--even if you don't kill it, then you'll at least do quite a lot of damage. Finally, against armies of MEQs, having that fist swat down 2-3 models at the end of most combats can be huge. Someone else has already mentioned instant-killing ICs, and I second that.
As to fists in furious-charging assault squads--while it still hits last, a s9 fist is insane--against monstrous creatures and vehicles it works really well. I'll go so far as to say it's better than meltabombs against nearly any armor value, though I don't want to do the math. Meltabombs probably still are better against AV 14, but whatever.
I like Predators a lot, too. An Annihilator with heavy bolter sponsons is mobile, dangerous to nearly anything, more durable than you think (though they get slaughtered from the sides and rear--keep them well back), and quite cheap, for what it can do. You almost get two for the price of a land raider. I do, however, agree with running several--they work best in groups, and you certainly have to have other armor in your list to use them well. A list with only 1-3 vehicles is going to be out of armor very, very quickly--you have to spread out the opponent's AT fire.
Finally, Tornadoes are phenomenal. They die very quickly, yes, but do enough damage to compensate. They rip light vehicles (which are often more expensive than they are) to tiny pieces, do a lot of damage to just about any infantry, and can take heavy vehicles out more effectively than any other unit that has a price within 60 points of what it costs. They also are very forgiving to use--if you set up poorly, it doesn't really matter for them--they'll be able to easily fix any mistakes, because they're so fast. They also save your other tanks--if an opponent has to waste a long-range, high-strength shot that will still only ever glance them in order to take one down (which happens a LOT), then your heavier stuff remains mobile and dangerous for another turn.
Sorry for how wordy this was, I get sort of carried away.