I usually buy the RapidSSL cert from NameCheap, $10.95/y or less for longer periods. It's single root (unchained) and signed by a very well supported CA cert. The only downside is no subjAltName (which lets you do both www.foo.com and foo.com).

All you really need to know is what the root CA is, because some certs (even expensive ones like Thawte's EV) are signed by newer CAs that aren't present in older browsers, mobile devices, etc. Also extra "features" like > 1024 bit keys often cost more.