I can only speak to the chassis I own/have owned: two MDTs (LSS-22, LSS-XL Gen-2), MPA BA Comp, and Tikka's chassis on T3X TAC A1. None of these fits the OP's $500 limit if buttstock cost is included but, given my experience with MDT, the Oryx would be top of my list if buying new.
The MPA stands out in that it feels "bigger" than the others, especially with its vertical grip (no, I haven't actually compared specs; that would be too sensible). I got it slightly used. The biggest attraction for me is the built-in ARCA rail, which allows me to slide a bipod or ARCA-clamp-equipped bag back&forth to best work with whatever prop I'm shooting from. I also have become a fan of vertical grips.
The MDT chassis are solid and functional and MDT provides top-rung support - the questions I've asked via email or their web-based contact form are answered within hours, with followup.
The Tikka chassis's big draw was (I just sold the it to fund another rifle) that it's a folder; otherwise, for me it was comparable to the MDT units. I don't understand why chassis vendors charge $150 or more for a folding option. It's handy for cleaning, but carrying the thing folded requires a case/bag deep enough to accommodate the "double thickness."