Clearly not from the Top 4 reference, but would be mightily funny if it was Saracens. Again.
I'd be very surprised if it was Bristol or Saints. The latter have been pretty quiet on the Salary Cap issue, but never looked to be over at any point based on their squad. For Bristol, they look to be over the most based on some of the rumoured salaries for their squad and their signings. However, Pat Lam gave an interview on The Rugby Pod where he was explicit in saying that any breach of the cap would land squarely on his shoulders; he's supportive of it and takes an active role in making sure the squad fit in the cap.
I'd be equally surprised if it was us in any meaningful other than a nuanced oversight like Haskell not being picked in the EPS and VAT on agent fees. We've been vocal in the cap being important as well as suggestions we're well under the cap (by Dai earlier in the year). I think Vaughan/Eastwood have since said we spend up to the cap, or as much as we can - however you want to interpret that (i.e. up to the cap because that's cap and so we can, or as much as we can as in it's within our budget which is lower than the cap).
Chiefs have been extremely vocal, so it would be so daft for them to be over the cap or doing something untoward. You'd think they'd want to be squeaky clean at the moment.
For me, i'd say it'd most likely be Sale or Bath. Be interesting to see what comes of it.