I use Ableton Live. It's the most immediate studio. It gets me writing music right away. I find that the best songs I've written are tracks I wrote in one sitting, so speed is important. That said, I'm running v6... they update yearly, and I don't have the money to keep up with them.
My favorite synth at the moment is actually Rob Papen's Predator. It's not the best-sounding, and it's not the most capable, but--again--it's something that I can quickly get excellent results from. You'll hear a lot of his instrument in my new music (perhaps 1/3 of it). Very tweakable.
My best synth (notice there's a difference) is the Virus Powercore.
It's a Virus. Do I really have to say anything else?
I also make ubiquitous use of Spectrasonics' Stylus RMX and Atmosphere. Both are excellent samplers... but that's really all they are: there's not much I can tweak. ...but they both sound incredible, so I use them a lot.
Lastly, I have GForce impOscar. ...A little synth with a lot of personality. I don't use it as much as I would like: he doesn't seem to play nice with the other boys. There's a lot of him in Black Mesa Winds, but not much in the upcoming album.
On the shopping list? PoiZone, Albino, and Surge. I like having as minimal a studio as possible, so I really don't want to over-do it... so I will probably sell impOscar and pick up PoiZone. The latter also has a lot of personality, though it's something of a one-trick pony (well, two-waveform, anyway), but I love the sound. Albino is something I used before my stuff was stolen, and--dammit--I want it back. Great, versatile synth with a smooth sound. Surge is more versitile, and I'm missing a nice digital edge in my music. I love the wave-shaping abilities of it. (Rather, I miss the wave-shaping abilities of Z3ta+, and Surge comes closest to it.)
I write my music on a simple, old Dual-G5 PPC Mac, running OS X Tiger. Yes, I kind of miss the PC for writing music (more plugins available)... but it wasn't worth dealing with Vista. I also wish my machine had a little more beef. But I'm a po' boy, and can't afford big steel.
For effects, I only use freebies, and I use them sparringly. Favourites include Ambience, Analog Delay, Blockfish, CamelCrusher, MDSP Compressor, MonstaChorus, SupaPhasor, and Multi Delay. If I can get away with using built-in effects (delay and what-not), I will.