Tippmann tippmann tippmann.
My field here had spyders at the start. Huge problems. They took a ton of maintenence, the kiddies would pull the bolt pins out and lose them, the frames would crack, every now and then a kid would just pull the spring out the back.
Quick strip features is crazy nice for your own gun, but for rental it's a big issue.
Model 98's never need to be taken apart unless they have a part break which is rare, if it ever happens, and if you make sure the barrels are clean they shoot pretty dang well no matter how much you put them through. I have a m98 as a primary woodsball marker myself for that one reason.
The biggest thing is that they are concealed. You can't take the bolt, hammer or springs out on the field and they never wear out. My tippmann has 100's of cases on it and the only thing I've ever given it was oil and a new detent every year every time I take it apart.
Tippmann are by far the best rental markers I've ever seen. A bit pricey upfront but well worth it in a quality rental fleet.