Brago, King Eternal is fairly unique among legendary creatures in that he has synergy with planeswalkers. Some other legends are good at defending walkers (i.e. angus mackenzie), or have cross synergy (i.e. zurgo helmsmasher and boardwipes), or just have a lot of colors to provide access to more walkers, and vorel of the hull clade has synergy if you can turn your walkers into artifacts, but in terms of straight-up synergy, brago is the king.
And what a king! His ability to blink walkers not only lets you reset the counters on your walkers, but you can also use them twice in a turn, leading to such absurd plays as Tezzeret fetching mana crypt and sol ring in the same turn, or elspeth 3.0 nuking all fatties and then making tokens, or jtms brainstorming twice, or karn exiling 2 permanents, and so on and so forth. Just about all of the walkers in his colors have nutty synergy with him, and his colors have some of the best walkers in the format to boot.
Tested last night, and this is actually one of my favorite decks I've built in recently memory, easily beating out Sidisi, Alexi, Brimaz, Sek'kuar, and even Marchesa. I only won a few games, but the ones I lost I made some pretty critical misplays that might otherwise have let me win the game. Having tons of planeswalkers was a lot of fun, and it has just enough interactivity to feel in control of the game.
I'm thinking of cutting Tamiyo, though. Despite ultimating her one game, the ability to + her twice a turn isn't that exciting, and frequently her second ability does nothing. By the same token, I'm not sure if thrummingbird or contagion clasp make sense here either, since they work better if I'm planning to ult a planeswalker, whereas Brago just wants to + and - them a bunch. Contagion engine is probably still too good to ignore, though, between the ability to annihilate people's boards with Brago and the ability to dump 4 counters on a planeswalker in a turn, which can pretty easily get them to ult right away.
Superfriends + Proliferate looks like a fun combo, I might consider it in the long run for my Roon of the Hidden Realm blink deck. I've played a bit with Brago too, and especially untapping mana-artifacts like Gilded Lotus is great. What's your MVP walkers so far?
I'm currently building a Sidisi-deck, and since you have the experience - and if you have a few minutes - could you have a look at my thread?
I wouldn't call it a combo per se, at least not compared to superfriends + brago. Given that roon doesn't blink walkers (and nor do most blink enablers) it doesn't seem worth going into without brago in the command zone.
Gilded lotus and co didn't make the cut since very few cards are terribly expensive, so the mana isn't really necessary. Getting to 4 on turns 2-3 is clutch though, hence the cheaper rocks.
best walker is tezzeret, by far. karn is also a superstar. baby jace isn't the nuttiest but he's amazing value for the cost. elspeth 3 is pretty nutty as well since she rarely hurts your creatures but nukes everything large out of brago's way.
I'm no expert at sidisi, I only played it a few times and didn't really like it. Not reactive enough for my tastes.
Have you considered the white and blue Parallax enchantments? Parallax Tide especially plays nice with Brago, either ramping by bringing your own lands back after a Brago trigger or by disrupting your opponent's mana base. I've frequently managed to use it to shut an opponent out of an entire colour.