2nd hand Rocktron Replifex is pretty good for the budget. I have no idea about the quality of current rack gear, but one cool feature of the Replifex is the ability to switch amp channels via a regular jack.
That used to be the case, but they've really picked up on the used market lately. They're still a decent deal, but nowhere near the bargain they were 2 or 3 years ago. It's hard to say what the best budget rack processor is without knowing the specific use. For a PA reverb, I'd pick a Lexicon MPX-100/110 since they can be found used for $50-80 all day long. I've used one for guitar and it works, but it's not great. For a guitar processor a v1 TC G-Major can be had for $120-150 and will also switch amp channels for less $$$ than the Replifex. It's much more clean/clear/modern sounding than the Replifex, so it may not be your thing. The switching knobs are also a bit fragile and a PITA to replace, so handle with care.