There's a big difference between who's the easiest to trade (Gardiner has the most trade value, hence is the easiest to trade) and who is most likely to return proper value in a trade.
I'll assume the second question is the point, 'cause it's the harder one to try to answer.
It may seem a bit strange, but I'll argue that Liles fits the bill.
First, the why not everyone else: Gardiner is hard to trade, as finding someone to give proper value would be tough. Same for Gunnarsson as he's not a piece people give up premium stuff for, but he's so cost effective and useful, it wouldn't be worth dealing him. Phaneuf is too expensive to get good pieces, but there's no chance the Leafs are in a position to disperse all his minutes among the remaining existing d-men (this one could be more debatable, but we talk about Dion enough already). Dealing Komi nets nothing if even possible and likely saves little (buyout next year should be 2/3 of 3.5 right?). Franson and Kostka are kind of wild unknowns. They'd be interesting in larger packages, but just figuring out what to ask for trading them on their own would be very hard. They're only easy to trade if you're willing to dump for little. Fraser and Holzer are perfect swing/depth pieces. You rarely get much for them, but you better have them. It's probably not too hard to find the right team that needs one of them enough to give proper value, but they're just such a boring answer, as the value we're talking about isn't all that much either.
Which brings us back to Liles. The cons on Liles are twofold: recent injury-related decline and a long (4 year) contract for an older player. However, he's got a skill that is always in demand, especially by teams looking for pieces at the deadline, but may be about to be squeezed out once Gardiner is able to come back (that does assume a few things, I'll grant). He's also got a deal that could be bought out after just two years if necessary. If he plays as he has for the next 20 games (negating the concerns that would've developed after last season) and Jake comes back and looks top 4 capable (making him worth less to the Leafs than to other teams), then Liles stops being a necessary piece for the Leafs.
And while we don't like his deal, it would take very little work to make it quite manageable even with 3 more years after this one. The last two years are 7m in salary (only makes 2.75 in the final year), requiring a touch over 4.66 buyout. If the Leafs just kept back half that in the deal, it require less than a 600k cap hit per year. This way the Leafs, for little burden, would share the risk on Liles' buyout. If he was worth it for even three years, then the acquiring team gets him at a bit of a discount too, seems like the kind of reasonable gamble you'll see around deadline time.
IMO, he's exactly the kind of fit that could get something of reasonable value back without being a premium asset that's always hard to give up.