I think I've developed a surefire sequence to turn anyone into a Brony. You don't have to watch all the episodes in chronological order, you just make sure the important moments are captured in order and get to the best episodes as quickly as possible:
1 - Start with "A Canterlot Wedding." It's easily the best, most mature episode of the series, featuring the best songs, the best visuals, and the best writing. It also doesn't spoil anything
(Twilight's ascension to royalty/becoming an Alicorn, Starlight and Discord joining the gang, the keys to the Tree of Harmony lock box, or the Changeling transformation). If you're going to get someone jaded to like the show, you need to hit them hard with the absolute best. They'll soften up from there. The absolute best of MLP beats everything I've seen so far in Futurama, so if you're doing an episode exchange with a friend, this will hit them hard.
2 - Stick to Season 2 following the finale above. Show episodes that really subvert the childishness of the show like "Putting Your Hoof Down," "Lesson Zero," "Read it and Weep," and "Ponyville Confidential." They all show real issues, avoid dimming to childishness, make it easier to appreciate more idealism later on, and they're all expertly paced and great fun. This is the show's golden years. Try to stay away from the Season 1 weirdness (The E/I days) and avoid episodes that require too much backstory to enjoy. The best episodes to start with are character-based, self-contained, and punchy. No Winter Wrap Up here. After enough letter-lesson episodes, finish with the Season 2 premiere, "The Return of Harmony" as this sets up the fight with Tirek later.
3 - We have not reached this part yet, but after you've exhausted the good episodes from season 2, try to ascend the scale quickly with the Pilot starter, any relevant first season episodes to what you'll see later ("Cutie Mark Chronicles" for Season 5 finale, "Call of the Cutie" for the filly's storyline, Trixie's debuting "Boast Busters" and "Magic Duel" for the season 6 finale), interjecting the lesser, more plot-necessary episodes with "The Crystal Empire," and try to peak with Tirek's fight in "Twilight's Kingdom" as fast as you can. You can skip "Magical Mystery Cure" unless the person is absolutely interested in how
Twilight became an Alicorn. Now that the golden episodes have passed you may have to start digging into the lesser-loved ones, but as long as you keep alternating the lesser plot-necessary episodes with grand slam ones, the lessers can be forgiven if they lead to something epic.
Have caution about overstating the importance of a character introduction episode. Canterlot Wedding gets a pass because it doesn't just introduce characters, it was single-handedly the single most defining event that turned everyone into Bronies. You don't need an introduction for Thorax, but you will need one for Discord. You don't need an introduction for Sunburst, but you do need one for Starlight. Let the curiosity of the person you're introducing the show guide which episodes you show them. If they're interested in seeing more of Luna, show them "Luna Eclipsed." But keep this guide in handy:
Twilight earned her wings at the end of Season 3 in "Magical Mystery Cure." DO NOT show episodes beyond Season 3 without mentioning she's a princess alicorn, or you will confuse the hell out of them. Follow up with Tirek immediately, as this is where her princess role is established, the castle grows, and the cutie map appears.
Discord becomes good in Season 3, Episode 11, "Keep Calm and Flutter On." If you plan on showing the Season 2 premiere, don't show any other Discord episodes before this.
Starlight joins the Mane 6 at the end of Season 5. In that same episode, you reference Sombra (S3 E1-2 The "Crystal Empire"), Nightmare Moon (Pilot), and Chrysalis (S2 E25-26 "A Canterlot Wedding"). There is a small reference to Flim Flam at the end (S2 E15 "Super Speedy Cider Squeezy 6000"), but it's not necessary to cover.
Flurry Heart appears in the season 6 premiere S6 E1 "The Crystaling." But it can be assumed the married couple eventually had a baby, and this premiere sucks.
The Changelings reform in the season 6 finale S6 E26 "To Where and Back." Thankfully the only new episode that came out with the reformed Changelings is the season 7 premiere S7 E1 "Celestial Advice" so this episode is not required for viewing the new episodes, but you must at least watch Starlight joining the team (S5 E26), Discord's introduction (S2 E1), reformation (S3 E11), and Trixie (S1 E6, S3 E5, especially and most importantly S6 E6 "No Second Prances") for this episode to make a lick of sense.
We'll see how this goes...