I've been playing a lot of Guild Wars 2 lately, and the big monster in the human lands is centaurs. I thought this was pretty cool. Usually, games go for goblins or orcs as the main monster, but not here. I like it. However, I do have a fondness for goblins, so they will always be important in any campaign I run.
Monsters I prefer as the bog-standard bad guys:
1. Goblins. The classic bad guy, for a reason. Sneaky, mean, and dangerous (but not too dangerous for 1st level characters). I mix hobgoblins in with them for extra punch.
2. Beastmen. Tough and also pretty darn evil, especially if you make them more like the broo of Glorantha or the beastmen of the Old World of Warhammer.
3. Gnolls. Also very tough, but great villains. Hyena-men just sounds dangerous. Add in the laughs at night while the party is camped and get the players worried.
4. Lizardmen. Tough and dangerous. I often pair them with yuan ti.