
Wow, a second consecutive tear-jerker from the Kitsis/Horowitz team, which is generally synonymous with humorous episodes.
The Others are coming, not tomorrow night but tonight! Three of the Losties must stay behind and shoot dynamite to wreak havoc on the Others, while Jack leads the rest to the radio tower. Just one problem: they can't transmit from the tower until the underwater Looking Glass stops blocking outgoing signals (for now we'll ignore the improbability of an underwater station being capable of doing such a thing).
Desmond's already seen this play out via flashes. He sees Charlie drown after flicking a switch in the Looking Glass, but his heroic deed will ensure rescue for the Losties (for now we'll ignore Desmond's thrice-repeated claim that Claire and Aaron get on a helicoper).
After learning his fate, Charlie composes a list of the five greatest moments of his "sorry excuse for a life" (his words, not mine). All five moments are illustrated in flashbacks, some of them very close to the essence of Charlie-centrics we've seen in the past. I won't step through them all, I'll simply say that it was quite an emotional journey from #5 through #1, with the last one reducing me to tears every single time.
Desmond makes a last-ditch effort to go in Charlie's pace, but he is denied (for now we'll ignore why Charlie chose to knock him unconscious instead of simply saying "thanks but I got this"). Charlie does reach safety in the Looking Glass and lives to see another hour but I'm going to honor him at this time anyway. The rest of this review will be comprised of several lists.
The following is how I rank the Charlie-centrics:
4. The Moth
3. Fire+Water
2. Homecoming
1. Greatest Hits

I hereby rank the acting performances in Greatest Hits as follows:
5. Neil Hopkins
4. Jorge Garcia (whatever, love you too, man)
3. Matthew Fox (we're gonna blow 'em all to hell)
2. Henry Ian Cusick
1. Dominic Monaghan *
* the scene in the boat, right after Charlie finishes the list, is perhaps his best moment on the series. It chokes me up every time to see his eyes welling up with tears and hear him say, "It's the five best moments of my sorry excuse for a life. My greatest hits."

I hereby rank the visuals/effects highlights as follows:
5. Charlie's shoe sinking in the water
4. Danielle's triggered explosion
3. Flashback of Charlie and Claire meeting (really felt like a scene from Pilot)
2. Aaron reaching up and touching Charlie's face *
1. The Looking Glass station (great design)
* Okay, so #2 isn't a visual/effect, but it IS a great decision by Stephen Williams to keep that shot in. It felt very organic, very real.
Note: deductions for the extremely obvious goof where a crew member's leg is visible when Hurley says goodbye to Charlie. I'm normally very lenient in technical goofs but this one should've never made it to the final cut.

There's no doubt Greatest Hits holds up extremely well, however it wouldn't be right for me to say I "enjoy" it every time. It's just a heartbreaking episode, definitely not something I'd want to see if I'm depressed.
Even though Charlie doesn't technically "flip the switch" in this episode I'm going to give it bonus points for significance anyway. If he doesn't volunteer to be the hero, who knows, the switch may never be flipped in the next episode. And if the switch isn't flipped, Season 4 suddenly looks very, very different.

I already mentioned the tear-jerker scenes so that's already covered. I'll add one more thing that illustrates my emotional investment in Greatest Hits: it may sound strange but just before Charlie dives into the ocean I take a huge gulp of air right along with him. It's partly my investment in the storyline, but also the fact that my biggest fear is drowning (yet I love water, go figure). I guess I just don't cope very well with underwater scenes.
The sad part is, one might think that the resolution at the end of the episode is reason to celebrate. The real emotional impact comes a bit later.