There are so many Isabel-centric episodes I love. Where do I even begin?

I'll just do it by season.
Season 1:103 -
Monsters109 -
Heatwave111 -
Toy House112 -
Into the Woods119 -
Four Square120 -
Max to the Max122 -
DestinySeason 2:203 -
Surprise206 -
Harvest207 -
Wipeout!208 -
Meet the Dupes (This is really a Lonnie episode but, hey, it's still Katie.)
209 -
Max in the City 210 -
A Roswell Christmas Carol211 -
To Serve and To Protect215 -
Viva Las Vegas216 -
Heart of Mine218 -
It's Too Late & It's Too BadSeason 3:303 -
Significant Others304 -
Secrets and Lies305 -
Control306 -
To Have and to Hold (hissy fit goodness!

)
307 -
Interruptus309 -
Samuel Rising (the return of the Christmas Nazi)
311 -
I Married an Alien314 -
Chant Down BabylonI still suggest that you watch all the episodes since this is a serial, not a series, meaning each episode is closely connected with the one before it. Isabel is practically in every episode too. There's not much of them. It's just 3 season's worth so it's not gonna be hard to watch 'em all.
Here's a tip. When don't want to watch the other's, I just use the fast forward button in scenes without Isabel.