With the addition of 4* Spidey Miles, I’d like to cast my vote for his yellow power as the worst team up available.
It’s already a pretty terrible power when used with the character but at least it accelerates the damage on his purple and has a passive that makes it decent. As a TU it just shoots a few web tiles on the board that do absolutely nothing.
Not if you bring 3* or 1* Spiderman but think of it this way, with the increase of 4* AI T/U (e.g. Red Hulk) it will be nice to see the AI use a useless T/U against us instead of the high damaging ones we’ve been seeing lately.
It’s still Elektra, who throws some enemy strike tiles on the board and then bails.
But Elektra could actually be useful especially against strong strike tile generators like OML. Miles is only useful if you bring a 1* or 3* character that no one uses.
Rags healing power, it’s useless instant garbage can for me.
Ragnarok Blue. You aren’t even on my team! How are you going to heal yourself when you aren’t even there! Riddle me this…
Invisible Woman’s old Green was the worst. She would throw out some bubbles then end the turn 
Eh, if 1* Venom counts as somehow making the Miles Yellow TU useful, then Rags Blue can be useful if you can create Green tiles to make use of the charged tiles (GSBW, Kamala) or if you can use those Charged tiles somehow (4*Thor).
But web tiles are even more useless than charged tiles for 97% of the characters in the game.
You can still plan ahead for Miles and the Rags’ 
though, even if you don’t think Thoress or the Spidey’s are worth the effort. Green charged tiles still do give more green ap than usual which is good for all the green damage users out there.
For cost vs return value, I still think Yelena and 2* Bullseye 
's are the worst. They’re just too expensive in cost to justify the little they do when they finally are able to be used. 5* team ups cost less to fire than Yelena’s Payback, and there are better ways to deal with special tiles than her.
The computer has been using Rulk’s utility TU a lot recently, I am sure to be consistent that the devs will ensure that Miles’ yellow will see as much use for them. 
What you’re forgetting, aesthetocyst, is that 85.67% of all quoted statistics are completely made up.
My vote for worst team-up is Yelena’s black. 13 AP to create 2 critical tiles (if enemy special tiles exist). You can argue that at least 4.5% of characters can use use web tiles, but if your match is taking so long you can actually gather 13 team-up AP and the only team-up you brought was Yelena, then you have other problems.
I occasionally have found a use for Onslaught, it works okay for teams with passive 
's, like the Blade/Daken combo. Daken’s matching green anyway, so why not? It’s not great , only good. Especially with Gamma Siphon showing up all the time as a AI TU now. Yeah, you’re right, it’s probably better to toss it in the bin to make room.
Also I know people love Twin Pistols for its targeting but any turn ending TU is a no go for me too.
Although Twin Pistols is an example of a TU that might be preferable to its original form because when that turn ends, it doesn’t leave the squishy Hood standing bare-ass naked in the front row.
I actually keep Repulsor Punch once in a while for when I’m using a team with no red power - it can be a good match ender when you’ve match-4’d and cascaded into 20+ red. I frequently forget I even have TU powers for some reason, so the increased cost doesn’t bother me since I usually end matches with a ton of unused TU points anyway.
It’s nice to mention that Flame Jet is the opposite of these.
Normally, Flame Jet takes 5 
to launch, and after spending that 5 green, THEN you use your reserves every turn.
Team Up saves you that 5 green so it goes straight into the damage hurt pile.
2* magneto 
Takes so long to go off, underwhelming effect for the most part, costs too much for too little imo.
I like to see Flame Jet on the other side because the AI will usually launch it even if they’ve got a big green AoE users. I’ll purposely try to protect it to keep bleeding them down.
I admit - I forgot Bagman (excluding him makes it 96.66…% unless I’m mistaken).
Still, your cutesy correction doesn’t really invalidate the point of my claim. So yeah, thread still over, for you 
Ragnarok’s blue seems pretty pointless as a TU.