I decided to revamp my Tier List for Best and Worst episodes of Lilo and Stitch: the Series.  For the curious, here's what I mean by each:

Best: If you're new to this series, the "Best" episodes are the ones to start with. Due to their smart writing and clever humor, the strengths of the series are clearly visible.

For a newbie: If you're new to this show, these eps are a great way to go.

Middle: These episodes fall somewhere inbetween "Best" and "Worst". While they're not without their charms, their flaws are more notable; they won't leave a newbie begging for more, but they're harmless.

For a newbie: "Middle" episodes are up to you, but try to see a lot of the "Best" episodes first.

Worst: I would not suggest starting with these episodes; they have weak writing and poor humor. The flaws clearly stand out in these episodes.

For a newbie: Take my advice and skip these episodes for now; watch them after you are well-initated (when they will do the least harm).

DISCLAIMER: Keep in mind that these are only my opinions. You are free to make up your own list. I will only rate the episodes of the show, not any of the movies (Lilo and Stitch, Lilo and Stitch 2: Stitch Has a Glitch, Stitch!: the Movie and Leroy and Stitch).

Tier 1: Best Episodes

Experiment 513: Richter
Experiment 254: Mr. Stenchy
Experiment 300: Spooky
Experiment 520: Cannonball
Experiment 601: Kixx
Experiment 619: Splodyhead
Experiment 383: Swirly
Experiment 032: Fibber
Experiment 199: Nosy
Experiment 158: Finder
Experiment 624: Angel
Experiment 323: Hunkahunka
Experiment 258: Sample
Experiments 149 and 150: Bonnie and Clyde
Experiment 319: Spike
Experiment 062: Frenchfry
Experiment 113: Shoe
Experiment 020: Slick
Experiment 029: Checkers
Experiment 319: Spats
Experiment 322: Heckler
Experiment 267: Wishy-Washy
Experiment 234: Shush
Experiment 285: Lax
Experiment 120: Snafu
Experiment 251: Link

Tier 2: In the middle

Experiment 177: Clip
Experiment 007: Gigi
Experiments 501 and 502: Yin and Yang
Experiment 303: Amnesio
Experiment 586: Tank
Experiment 509: Sprout
Experiment 627
Experiment 025: Topper
Experiment 228: Melty
Experiment 604: Houdini
Experiment 523: Slushy
Experiment 344: Dupe
Experiment 010: Felix
Experiment 222: Poxy
Experiment 151: Babyfier
Experiment 608: Slugger
Experiment 355: Swapper
Experiment 133: PJ
Experiment 505: Ploot
Experiment 277: Snooty
Experiment 540: Phoon
Experiment 607: Rufus
Experiment 276: Remmy
Experiment 223: Glitch/Experiment 600: Woops

Tier 3: Worst Episodes

Experiment 375: Phantasmo
Experiment 606: Holio
Experiment 345: Elastico
Experiment 613: Yaarp
The Asteroid
Experiment 602: Sinker
Experiment 297: Shortstuff
Bad Stitch
Experiment 360: Drowsy
Experiment 089: Skip
Experiment 210: Retro
Experiment 248: Belle
Experiment 316: Morpholomew
Experiment 128: Bugby
Mrs. Hasagawa's Cats/Experiment 262: Ace