Below is the main things I will suggest you to check
1. Infrastructure is the main thing you should look.
Whether they are having SSD drives or not
Details of the Data center where the server's are located , Whether the DC is providing cheap quality products etc
Details of the Software Running in the Reseller Servers like Litespeed, Nginx Reverse Proxy, Mysql on Seperate SSD drive, Mysql versios etc
Whether they are having any Raid like Raid 10 or raid 1 etc
Spec of the Reseller Servers to confirm it is having latest CPU and enough memory etc
Great Network Support and possibily IPV6 support as well now a days
2. Second most important thing is Support.
Make sure the support is having 24/7 real support and check and confirm the response timing for the below and see if they are meeting the SLA
Live chat
Tickets
Phone support
24/7 Inhouse Monitoring of the Services and if possible to have some custom checks url site check for re-sellers for their main site.
3. Third important thing I will say is Proper Backups for the data
See and confirm the backups are continuously monitored and backups are not in the server itself. So that you can get the account's backup even if the server is badly damaged.
Another thing you need to consider is you should get the hosting from the location where most of the targeted visitors will be for get hosting from a company having US data center if your targeted visitors are from US.