After running through a few matches with each of the four offered, I’m partial to CIRC. Kept me grinning the whole time, anyone else liking the new A.I. companion so far? 
Lumu all the way
My boy Fret is the MVP in my book.
I loved Lumu
CIRC!
my favorite line is: “HAA HAA, no one will ever believe this!”
i cannot WAIT for more companions, I’m hoping they take a chance and add different languages. Or MAYBE the Noble 6 team VAs. Reach Firefight-esque.
To be honest they all were great, they all had funny one liners. But I stuck with lumu the most.
I really wish for a virgil ai like from odst! Just silent and menacing lol with beeps
I really liked Circ!
Circ reporting in
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> My boy Fret is the MVP in my book.
I like Fret because he reminds me of the AI Arthur from Journeyman Preject 3
Tie between lumu and Fret. Butler is also good.
Team Circ here.
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> I really wish for a virgil ai like from odst! Just silent and menacing lol with beeps
Thatd be the best, just chucks in a little “welcome new Mombasa… oh”
One thing I find I’m hoping for, is that you can change your A.I. companions shape as well. While I think CIRC is kinda cool, I want that personality in the pyramid. Will the customization allow for that?
I liked the one which had nightmares about the needler. 
Butler is great too. Kinda reminds me of Shaxx from Destiny.
CIRC is my favorite but I also use butlr sometimes.
I am a Circ enjoyer
i don’t think i love any of them, even including the two leaked, but i spent most of the time with FRET, i liked how he was concerned for his own safety a bit more, not necessarily drawing a parallel with CE cortana, i just like how he’s all, “well I’m glad i didn’t get MY head blown off.”
i actually found LUMU and CIRC hard to discern from the rest of the spartan chatter, and with BUTLR being based of Black Box, i had kind of a hard time palling around with an ONI AI. i found FRET casual enough that i could notice when he was talking… though having just now looked back at the AI’s, it would seem FRET is played by a recent spiderman cartoon voice actor, didn’t learn or look up anything about the others, but interesting.
so, I’d say FRET was my favorite of these, but i can’t wait to see more.
LUMU because she cuts to the point. A bit cold/robotic too which is a good thing in competitive scenarios.
Butler would be my second favorite.