I like to play all-instant speed or almost all instant speed to make a judo-focused strategy really work. I found having too many sorcery-speed things causes the unfortunate situation of me having to pre-emptively react to the changing board, tying down my mana that could be held up for reaction on opponents' turns.
With that in mind, the color identity becomes way more important - with such a limited card selection, access to a color becomes less about "having the best of these effects" and more about "having any access to these effects at instant speed at all".
As an example, one of my failed attempts at a judo deck was
Surrak Dragonclaw. While it was able to react to the table and effectively discourage people from attacking me in the early and mid-stages of the game, it was severely hampered by my inability to deal with rapidly escalating board presences, and with my lack of options to do the same.
perilous vault and
oblivion stone look like low-CMC cards with an instant-speed option, but in practice they're basically CMC 8 and 9 cards because the moment they hit the board, everybody wants you to pop it on their turn so they're ahead of the rebuilding. Likewise, there's just not a whole lot of cards like
decree of justice outside of white that you can dump large amounts of mana into a game-winning board presence during a lull in the action.
White is mandatory in my opinion. Being able to board wipe when players develop overwhelming boards is necessary, and white has
rout and
fated retribution.
Dismantling Wave also deserves a look, especially if you don't have green color identity for
fracturing gust. At the same time, it's got plenty of cards mentioned above and more that can turn an empty board with no tapped lands into a three-turn clock for the table in the late game, giving it plenty of closure potential.
Blue is very helpful because it has a lot of game closers like
time stop, and the much-hated
cyclonic rift is one of the very few instant-speed board cleaners available. I'd argue counterspells are against the nature of judo however; the point is to let your opponents attack into each other and punish them when they attack you. Removing
Inferno Titan before he is able to kill opponents' creatures and hit them for six means more pressure for you to find a way to deal the necessary 120 points of damage to win the game. Despite the fact that I've never had a blowout game with
portal mage, I'd still argue that these are the kind of interaction effects that define the judo playstyle.
Green has ramp, both as instant-speed cards like
harrow, and one-mana cards that don't gum up your instant-speed mana. There's a few really nice cards like
chord of calling that can fetch a win condition on a lull. Since you should be playing white anyways, having access to
fracturing gust is almost worth the price of admission by its self, but by and large I'd say the greatest value comes from less reliance on signets for your early mana boost.
Red has a few donkey-punch cards like
Reiterate and
Wild Ricochet that can really turn games around if people walk into them. Blue has definitely gotten better at this identity since I last ran red in a Judo deck, but if you aren't running blue then you can still run a lot of these effects through red. It also has access to instant-speed and low-cost board cleaners through damage, which while not always the best choice, is still acceptable if you're limiting your card pool to be appropriate to judo playstyle. Even the humble
comet storm can do a pretty good job of punishing an attack and cleaning the board of must-kill threats all at once when you're pumping 10+ mana into it.
Black is a little weak by its self, there really aren't any mono-black cards that really stand out as great instant-speed turnaround cards. Blue-black however has a lot of great picks, from the infamous
notion thief to the great utility of
ancient excavation. There's probably a bunch of other good picks with other pairs but to be honest I haven't actually ran black in a judo deck yet so I wouldn't know.
With all that said, here are some of my personal thoughts on instant-focused judo commanders:
Surrak Dragonclaw - As I mentioned above, he really suffers from not having access to white. The moment somebody casts an
avenger of zendikar it's all over; there's just not any instant-speed cards that can compete with, or answer, such a board-changing threat like that. That being said, it was a while ago when I built him so maybe there are a few new options that open him up as a commander more. The end-game plan of casting him end-of-turn and pumping him with
Kessig Wolf Run - or even just the public threat that you can do this at any time - is very powerful though.
Derevi, Empyrial Tactician - I could never get this commander to work well at this strategy - Judo mostly works by making yourself a terrible target to attack by punishing the first person who comes forward to try to stop you. Derevi however basically says "I'm always going to threaten the table with my stupid shenanigans as long as this player is alive", which to the table means the only solution to Derevi is to kill the player. Judo doesn't work so well when the whole table attacks you.
Ephara, God of the Polis - My most successful build. Having a commander that doesn't demand to be killed - and is pretty difficult to get rid of anyway - makes up for the fact that it's not instant-speed, but I can see this build not working so well if your playgroup tends to make moves to set up a win at turn three or four - since that's usually the turn you'll be tapping out to set up your card engine. That being said, being able to cut card draw effects creates a super dense pile of interaction in the deck that basically ensures the full hand you always have is full of answers.
Samut, Voice of Dissent - I haven't made this one yet but I've considered it. As I mentioned above, Blue isn't extremely necessary for the instant-only play style, though the card pool is a little bit weaker without it. That being said, the Wolf Run win path is super-charged here, being able to one-shot a single player in the mid-late game - and more importantly - threatening to do so at all times if someone falls behind.
Chromium, the Mutable - Another one I haven't built but it seems strong. Basically take the very solid blue/white package of Ephara, add some card draw effects, and a few of the best black/white and black/blue cards. Similar to Samut, being able to threaten a player with commander damage if it comes to the endstep before yours with seven mana up is strong, and the extra reach provided by cyclonic rift and time stop help close games.
Here's an older, not-so-updated list I keep for reference on my Ephara list:
https://archidekt.com/decks/184072#ephara_all-flash