By transaxle are you referring to the rear end unit sans the hydro pump? The Large Frame Bolens ( HT's or the 1250-HT 23) are the largest physically which would lend you to believe the strongest . The Duratrac's and " Eliminator's" are a bit smaller in size. As far as strength I think the rear end in all of them are extremely strong well built units, starting somewhere in the HT18 to HT20, Bolens decided to precision cast some of the rear end components and the bull gears were prone to failure, I believe they started heat treating them and this corrected the issue.
As far as hydro pumps they used Eatons and Sundstrands, both with pros and cons, I think consensous is the Eaton is better liked, perhaps stronger but personally I like the Sundstrands over the Eatons , thou admitting the 2sp rear ends with the Eaton pumps are hard to beat