My vote goes for Peasant Magic. For those who don't know, it's a nifty constructed format which goes like this:
- Usual 60-card decks are used
- A deck can't have any rares and more than 5 uncommons
Since we're playing online, it'll actually encourage us to make some fun decks instead of making some OP shit worth 100's of $$$ in real life.
While I like Pauper (all commons, but essentially a very similar format), these types of formats require a banlist a mile long or else some seriously broken shit starts happening. I'm pretty sure with just five uncommons and the rest common I could make a deck that can easily kill on turn 2, and is almost guaranteed to shove 20+ points of damage in the opponent's by the third turn with no bility to be interacted with by any nonblue/nonblack deck. There are a lot of busted-ass commons out there, and five uncommons is good enough for two Tendrils of Agony and three Demonic Tutors (if I even want to run those; 4 Demonic Consultation might already be enough).
I don't think any Highlander formats would be a good fit either, mostly because it's so unfamiliar to a lot of people. Not just the rules like the "command zone", "general tax", "general damage", and 40 starting life...but that deck construction is COMPLETELY different than normal Magical Children's Cards. Most people won't know how important mana acceleration is, or have encyclopedic knowledge of all the different "redundant" spells that allow one to get around the . Also it's very counter-intuitive in how some spells that are great in normal Tragic garner nothing but failure in Highlander. Plus, those types of formats are made for multiplayer.
My format pick would be one of the following:
1.) 60-cards with a gentleman's agreement. Don't bring a Spanish Inquisition deck you found on Stormboards, don't recreate the magic of "Combo Winter", no Stasis decks, and the like. Just use good judgment in what BROS will enjoy playing against.
2.) Legacy with a gentleman's agreement. Don't go to The Source and copy down a Deck-To-Beat. The above option is probably strictly better, but I don't know how much you trust people's judgment, and this option eliminates a lot of the egregiously stupid things. "Uh, you mean Necropotence is good? Whoops!"
3.) Choose Your Own Standard. This one is complicated, but really cool in practice. Each person makes a deck, but they are restricted to using cards from one core set and two blocks. The core sets allowed are 4th Edition onwards (Alpha, Beta, Unlimited, and Revised are too problematic) and all blocks from Ice Age onwards can be one of the two (early sets [Arabian Nights, Antiquities, The Dark, Legends, Fallen Empires] weren't designed with the block structure; sorry, no awesome Floral Spuzzem stacks). Lorwyn/Morningtide and Shadowmoor/Eventide can go either way in terms of counting it as one block or two separate blocks and fuck Coldsnap. The only banned cards are those that were ever banned in Standard, which is only a real problem for Urza's and Mirrodin blocks (Zendikar block is the only other one, with a mere two). It's a great format that plays like regular Magic and allows people to play with a lot of their favorite cards. The downside is, it can be kind of complicated to enforce, but this shit won't be DCI sanctioned so a few mistakes here and there are no big deal.