Most liquid, best & cheapest instrument by far IMO CBOT 10y or .. if you got bigger balls.. 30y bond futs.
best bet no doubt is ZN, but some people (cnbc viewers) don't know how to open a futures account, or they are afraid if they do they will accidently end up with a carload of corn on their front lawn. so second best bet is options on a long etf which is usually not shortable.
I think ZN is the most heavily traded contract on the CME. What is that ETF that is long the ten year? TFT or something like that. It is also heavily traded but can't be shorted, but the options are pretty liquid. Depends on your time frame, do you really want to pay a premium to buy the puts? Takes a big bear to do that. Selling the calls? I suppose if you are confident and right on the strike and expiration. Why do stock traders even ask these stupid questions? Are they afraid of getting their hands dirty on an exchange that also trades cattle (or use to)? Just short ZN and be done with it, that's what everybody else does
The yield on a 3 month loan 5 years from now increased 35bp in the last couple weeks, EDU19, the Sep 2019 Eurodollar contract. If you can trade futures you can trade these contracts. At 25 dollars per bp. per contract you can tailor your size and there is great flexibility for expressing your ideas on the shape of the curve as well. Not so easy for small retail having to short ZB and buy a bunch of ZT (hindsight bearish steepener that worked recently) ,if things dont go right , that could be painful , or at least shake you out of your idea.