The one they have the most fun with.
Seriously, that question is so wide open. Maybe I'm crazy, but it seems like you've got things the wrong way around. Find out what your friends would like to play, then find good builds for them.
Enjoy hitting things? Hard to go wrong with a pure barb or ftr. Or a monk if you prefer punching things. Pallies do less DPS and are more stat-dependent, but they also have better self-sufficiency & survivability. And few things do more DPS under the right circumstances than a backstabbing Assassin...
Like the classic S&B tank? Ftr or pally (or both).
Want to be a healer? Plenty of good cleric builds out there who do a lot more than just heal.
Like arcane spells? Go sorcerer for a pure DPSer or wizard if you prefer the CC / instakill spells (Necro, Enchantment, etc.).
Wanna deal with traps? Any number of rogue-based builds fit the bill, either pure or mixed with ranger, wizard, or even bard.
Enjoy the pew-pew-pew? An artificer, Arcane Archer, or Mechanic fit the bill nicely.
Prefer a jack-of-all-trades? A good bard is always welcome: buffs, heals, CC, and some DPS if you build it right.
The point is there are a lot of solid builds out there for first-lifers. Heck, there are threads dedicated to them. Asking something as generic as "what's the best build?" is just silly.