Best exterior paint?
Per the above, make sure the prep work is done properly.
Also, as per the above, Glidden makes good paint for some of their upper lines. For example the Endurance line is very good, it was rated very high by Consumer Reports, but it also costs about $10 more per gallon than their low-end lines.
Most painters are committed to 1, maybe 2, manufacturers. There are benefits to that loyalty in terms of rebates, discounts for volume purchasing, etc. Your guy may be doing it for that reason or his bid may have been based on the lowest price paint he could buy at the time & it turned out to be a low-end Glidden paint.
Found out which Glidden paint line it is & post back.
Being on a very tight budget & wanting to use the best exterior paint do not exactly go hand-in-hand. However, in the scheme of things it may not be that far off given this is painting. Say the house requires 30 gallons of exterior paint. The cheapest you can buy is probably in the $16/gallon range. That's $480. For $300 more ($26/gallon) you can buy a very good exterior paint.
I don't know your situation, but assume the overall cost (depending on prep requirements) is in the $3,000 range, an extra $300 for a very good paint is only 10% more. Given that painting cost is mostly labor it is usually better to pay more for a better paint since it will outlast & perform better than the cheaper ones.