Japan's UU and UA are both tied to warmongering. If you can find iron deposits near you when you get to Samurai, you're in luck. They can chew up almost anything the opposition can throw at you, without losing a beat due to the UA.
Samurai scare me (having never played as Japan, and knowing Oda's temper), but they're still basically just longswords with Bushido, and unless they catch you in open ground will often to their non-UA counterparts. As has been mentioned, bushido is a good ability for killing things, but as most players actually want to keep their units alive, it's not nearly as useful to a player as it is to the AI. As a unit per se, it's not as strong as the Berserker, I feel.
Do note that Japanese units can still die if left unattended with low HP. That's the Japanese UA's drawback- You might get a shock when playing other civs due to the lack of Japanese UA, so I won't recommend Japan to an absolute beginner- might give him/her the wrong habits with regards to unit management and tactics.
Also, as a warmonger civ and especially when relying on bushido, a Japan player is likely to get an unpleasant surprise when playing multiplayer rather than single player, due to the AI's difficulty in using ranged units well. Bushido isn't a lot of help against players who know to take down every damaged Japanese unit before it gets a chance to attack.
Another Civ with amazing upside is Persia. I'm a little surprised Persia isn't getting more attention.
Persia is probably not a good starter civ. Like Siam it has a UA that relies on management to work, and the civ is most effective when managing happiness not just to reduce unhappiness and expand, but to maximise the rate at which you get Golden Ages without expending GPs. I suspect it's overlooked because people prefer expansion and city-development strategies over those that maximise excess happiness and spur Golden Ages. Much like Siam, indeed: the UA's great, everyone knows it's great, but for the most part they don't play with CSes so it doesn't really register.
Okay between France, China and Rome... What should i play? as a beginner? if you guys say china then whats the second option? Rome or France?
My vote would be for France, due to the flexibility of its UA, the simplicity of its UUs (they're just the standard version with a strength bonus, that don't require much special use or attention), and the fact that they get social policies quickly - which allows you to experiment with that aspect of the game. I think you probably want to avoid being tied to a particular style of play as a beginner, simply so you can explore more of what the game has to offer and decide, once you've settled on a playstyle, which other civs might complement it better.
Having said that I've only played China once for the achievement, and haven't yet played Rome; both seem fairly flexible civs as well. I would say that having a moderately early, strong UU (like the Roman Legion) is probably less important as a beginner, because you're unlikely to be playing either for very rapid domination, or needing to defend against early attacks at the lower difficulty levels.