Humans are nice.. they're pretty diplomatic, yes. I rarely play them, but then aim for a New Systems Commonwealth (from Gene Roddenberry's Andromeda) kind of scenario with them I don't destroy empires, just play the good guy.
If I want a relaxing game, I usually play the Teekans. Even with a generally hard setup, they can practically buy the whole Galaxy. All of their Bonuses, the Other section and special government (Mercantile Guild) are serving one purpose: get riches...!
With faster cosntruction, higher migration rate and unmatched Civilian fleet they can secure conquered colonies much faster.
Larger civilian ships makes PreWarp expansion easier, because you don't have to cripple the cargo space on Freighters to add the warp drive and reactors.
Even though they have smaller military ships, with that much income and faster construction they can build and maintain at least twice as much ships as any other race.
They can almost constantly crash research until tech level 3-4.
Also 50% of their race-specific victory conditions are trivial: Start the Fewest Wars (25%), Highest Private Revenue (25%)
They're the ultimate race for me.