Cat 3406! If only there were skoolies out there equipped with these, I'd be running one of them!
ok, now that i'm being serious...
You'll likely get lots of varied opinions on this. Depends on who you ask.
I like Internationals. Always have.
I think how its maintained and its power rating are more important than who built them, but that's just my opinion.
My bus has a DT466 rated at 195hp. Its a 40 footer, and I wouldn't want any "less engine" than what I have, I can say that.
Some people hate on Cats as they can be bit pricier to repair, but I know some truckers that LOVE them.
The Cummins are a very popular engine. I have reservations about going with something as small as the 5.9's but there are plenty out there running them. Same with the dt360 in Internationals. The Cummins 8.3 is a really good one to go with. Lots of praise for those all around for the most part I'd say.
If you have the cash to spend, then Crown put some heavier duty engines in their buses. An 855 Cummins is just awesome. Long Haul power in a school bus! Most of them have Detroit Diesels in them. I like the sound of the old two strokes but I wouldn't want one personally.
Growing up, my dad was an old school trucker and he always hated anything DD. I guess I inherited that. Doesn't mean I won't be rocking a two stroke DD powered MCI one day though.
Take all this as just one guy's opinion as that's truly all it is.