I've been really pushing my rendering time....(Im a mental ray user)

basically I start with default light......till get my vegetation worked.....making sure im getting rid of all procedural patterns from vegetation...

then I pick my enviorment/sky/.....I tend to like atmospheric spectral stuff........from out of the box.......(godrays, spectral sun and sunsets).....

BE Care full alot of the skys automatically have renderosity chosen.....I immediately dial it down to like global ambience with preview setting.....make some quick checks for atmospehere and haze.....and direction of sun...

and start to work back up the ladder....doing tests at preview to broadcast to superior to ultra...

eventually I set off a renerosity render at ultra......with a 4K resolution overnight (8-20 hours)

if you want the good look you got to work for it....