Bornmad,
This topic is SOOOO subjective, it really depends on your tastes, playing style, and objective. I have probably spent as much on effects through the years as I have on guitars, so I'll give it a shot.
Chorus : the older Boss units (far from toys, look at all the pro pedalboards with Boss stuff) like the CE-1 offer a thick, very EFFECTED chorus tone. Other cool pedals include
the Danelectro CoolCat and the old DynoMyPiano. Far and away the best, IMHO is the TC Electronics Stereo Chorus/Flanger, but this puppy is MUCHO dinero. I have one which I got for a tremendous deal, but I would definitely pay full boat for another one if it needed to be replaced.
Tremolo : Amplifier trem (think Fender) sounds the best, but of course outboard units are more versatile for live playing (trem into delay...really cool). The Diaz Tremodillo is pretty neat, as is
the Tremulator, the Voodoo Lab isn't bad, neither is the Boss. I have a rack-mount Peavey Valverb (probably their coolest piece of gear), that covers this category and the next as well.
Reverb : Why would you want a reverb pedal? Well maybe you play live on stages of questionable stability with drummers whose hairy knuckles drag on the floor and the spring reverb in your amp sounds like Godzilla is right outside the club. Or maybe you just want the different sounds offered by digital 'verbs. That having been said, I use amp verb and an outboard (see above) for my Bassman which has no 'verb. I haven't tried the Boss or Digitech floor units, but I'm sure they're not too bad for guitar use.
Wah : The newer Dunlop wah's are a different inductor and a different circuit from the older ones, which I think are copies of the older Crybaby circuit. The newer ones seem to have a sharper "notch" to the filter which I don't like, but once again its a matter of taste. The original Vox or the new Fulltone are very good, but a lot of people really like those big '70's bell-bottom Morleys.
Delay : Really depends what you want to do with the unit. Do you want a clean delay to play repeated figures or do you want a more tape delay Echoplex-y sound. For the former, the Boss DD pedals or the older Digitech Multiplays will deliver. For the latter,
the Danelectro Danecho, the Korg Dynamic echo and
the Akai Headrush are digital units which aim to simulate tape delay and sound pretty good.
The Memory man pedals are also very cool, albeit clunky, units.
Flange : See chorus for my personal favorite flanger, which is a very clean unit. But if you want jet-like flanging, the Boss works pretty well. A friend of mine had an old Austin flanger, though, that sounded as if you were being sucked into a giant space vortex, or maybe that was the Dos Gusanos?
After typing all this, I have to pause and say that my views (and everyone else's opinions, as well) are really all just a giant load of crap when it comes to a topic as personal and variable as this. They are all valid for each of us in turn, but you really need to get out and try stuff out, then take it home and try it some more (buy from someone with a good return policy if you can). I've gotten some really good sounds out of
a Zoom 509 modulator pedal, but tons of people bash Zoom gear as being universally turd-like. People praise the Ibanez Ts-808 overdrive, but I've used one and its just not worth the "vintage" price for me to buy it. I'm not saying it wasn't GOOD, though. So I could have saved myself a lot of typing and said "try for yourself", but I like talking about this stuff almost as much as I like playing through it, so here we are.
I WILL say, at the risk of being flambeed, that nothing really sucks, there's a use and purpose for everything that sits under my hyperactive toes or on my never dusty pedal shelf.
peace.