Whatever decently high-end jersey you can get at 50+% off.
Cheap jerseys work fine if you're looking for a mid-weight jersey. Basically, they're a synthetic shirt with pockets. For a ride on a moderate day, they work great.
My "best" jersey is one of the LG pro carbon jerseys that I got for $55 from AeroTech Designs. Its by far the lightest & coolest jersey I have, I so lightweight it feels like you're riding without a shirt. Definitely my favorite jersey for riding in 80+F.
For example (S in orange is $64, the others are $160):
Louis Garneau Elite Carbon Jersey | Competitive Cyclist
I use "best" in quotes because I have an assortment of jerseys.
1 lightweight (LG pro carbon)
2 midweight (Descente & cheap ebay)
1 long-sleeve thermal
I wear them all about equally. They're all comfortable, its just a matter of matching the jersey to the weather.
I'm all for spending $$ on cycling clothing. My Assos bibs are dramatically better than my cheaper bibs. Jersey's aren't as critical an item. The main differences are in jersey's designed for temperature extremes. Buy one that fits you, and fits your needs.