The worst is 2H -> 1H. It's a shame, it's the best way to give incentive to twohanders to use shields.
That's not secondary mode, that's just using a 2h/pole with a shield. Not the same thing.
Although I do agree that "bastard mode" and "shield&spear mode" should be buffed a bit too, mainly the speed nerf on them.
And I personally find situations for most of the secondary modes. All the ones that change wpf groups are good (1h --> 2h or halfswording), as are the "cut to pierce/blunt" too. Not usually better, but for ambushes, tincans or situations you think you'll have a higher chance to glance, it's sometimes usefull to change. The ones I don't think are that useful are the pierce --> blunt or blunt ---> pierce secondary modes. Most (all?) of them are less damage, so not very useful unless they give a speed bonus or knockdown. Even then, not that useful.
Although we DO have to realize this is SECONDARY mode and should be less usefull than the primary mode in most sitations. Or as Ptolemy put it:
Protip: If the alt-mode is the better mode, it's no longer the alt-mode, the weapon just defaults to the wrong mode.