Three seasons are really not hard to watch (seriously, it took me about 4-5 days to blast right through them, but then again, I'm a marathon queen--I've watched entire seasons in 1 day before).
And I disagree with the decline. Season 3 was amazing, IMO. Season 2 had issues because of the Writer's Strike. Sylar is far more prominent in season 3 than he was in season 1.
But if you want to just watch episodes with a ZQ/Sylar-focus (his character is easily the most fun and multi-dimensional on the show), there's only 47 episodes that he was in out of the total 56.
If there's any episode that is known as the 'hook' episode, it's episode 1x09 - Homecoming. His first episode was 1x08 - Seven Minutes To Midnight, but all he does is sit at a diner in the shadows, show his cracked Sylar watch and can-open the waitress' head as she uses the can-opener. You don't see ZQ's face (once again, obscured by shadow) until Homecoming, and even then you only see his eye until his hat is taken off at the end. This episode is very necessary, however, to show what the series is about as well as to introduce Sylar. The first episode that really showcases ZQ as an actor is 1x10 - Six Months Ago, in which you get to see the Gabriel Gray part of Sylar (the watchmaking prehistory). 1x11 - Fallout is where the events of Homecoming continue and features some amazing scenes between Sylar, who has become totally insane, and Noah Bennet a.k.a. Horn-Rimmed Glasses.
Probably ZQ's best or most memorable episodes were:
Homecoming, Six Months Ago, Fallout, Distractions, Parasite, The Hard Part, How To Stop An Exploding Man, Kindred, Truth And Consequences, The Second Coming, One Of Us, One Of Them, I Am Become Death, Angels And Monsters, Villains, The Eclipse pt. 2, Dual, Shades Of Gray, Into Asylum, I Am Sylar & An Invisible Thread.
I think you can more or less piece together his journey with these. The Gabriel episodes (if slightly sociopathic, dorky watchmaker with mother issues that make him want to be "special" is more appealing to you than the absolutely delightful skull-slicing "magnificent bastard" unkillable serial killer) are Six Months Ago, The Hard Part (probably ZQ's best acting!), Villains & I Am Sylar (also one of his greatest performances). Shades of Gray goes a long way to show just why Sylar has more layers than just the "Sylar" image he puts on--it basically shows a mirror image of himself in his father and what he doesn't want to become.
Here's a fanvid that is pretty much The Hard Part and Fallout/the rest of season 1 in a nutshell (with a little help from Queen): http://www.youtube.com/watch?v=DfTDs44-n5c
(God, I love his facial expressions in The Hard Part.)
And anything Sylar/Noah is pretty much gold.
If it helps, George Takei and Nichelle Nichols have recurring roles on the show!