A couple of years ago I would have said Wawrinka but nowadays probably Thiem - Dominic uses his power more willingly when he's confident, which he has been in recent months.
Interestingly, the ATP did an analysis article some weeks back that stated Nadal has a faster average backhand speed than both Wawrinka and Thiem, but that's probably because Stan and Thiem use a lot more sliced backhands than Rafa as they have one-handed backhands, which brings down their average speed. If we just compare topspin backhands and flat backhands, which are where the power shots usually come from, then I'm sure the results would reverse.
Thiem hits his BH harder and with more spin than Wawrinka, that's what the numbers say.
However, there's no doubt Wawrinkas BH is a much stronger shot overall. Much more consistent, particularly his ability to contruct points off that shot is second to none.
FH is kind of the opposite. While Wawrinkas FH is by no means a bad shot, is still has nothing on Thiems which has become one of the most dominant strokes on tour, on every surface.
Practice sessions are not indicative.
Some players will hit and practice conservatively, others will hit harder and riskier than they would in real match conditions. (eg Nadal)
But it is pretty close, Stanimal seems to be able to unload with heavier shots more consistently.
Here's again another thread where we discuss who hits harder. Aren't you all aware that there's no clear answer and it's no use dwelling on who has the better whatever because it's only one dimensional. The best solution I can offer to you is to ask the ball hit by these gues how much damage their backhands do on the balls respectively. Ask them how much they hurt when they are hit by the two. They'd tell you more reliable information, though we'd still be unable to solve the problem completely. As far as I know, they both hit their backhands extremely hard with heavy top spin and they have similarities and they have some of the best and most beautiful backhands on the tour today. Of course they have diffeent motions which give them some advantages and disadvantages. There's no such thing as ideal backhand. As with anything, you'd likely to find both merits and demrits, so we can't say which is better or harder for that matter. We can analyze as much as we like and present about the advantages and disadvantages of their bakchands. I see nothing wrong with that, but please give me a break if you continue to discuss who hits harder. The balls would know better.