That's easy. Go with the classics: Blow up the entire hotel. (just in case)
Fast, Efficient, Effective.
lol I really studied this question. I came up with all sorts of creative ways (dressing up as a chef poisoning his food, crawling through air vents) but none of them seemed realistic they seemed like a bad spy movie.
Have a package delivered to a room that you booked but have it under a false name with an automatic timer set to go off.
I'm basing this question on the assumption of course that I was a iron skinned homicidal highly trained assassin who cared nothing about innocent people.
My story (listed above)
Blowing up the hotel causes massive attention, I really doubt your contractor would enjoy risking being in the spotlight. An explosion like that would lead to a huge investigation and there's a good chance you would get caught, considering the scope of the situation and the amount of people that would need to be involved to blow up an entire hotel. Besides getting enough explosives to blow up an entire hotel is incredibly implausible, not to mention even if you did, you have no guarantee if the politician dies, he could survive.
And it's very hard to get that amount of explosives into an hotel without getting caught, not to mention eye witnesses. It's a bit supicious if you bring hundreds of bags into the hotel, and yes you would at least that to be able to blow up an entire hotel. This isn't the movies you know.
So Fast? NO. Efficient? NO. Effective well, if you did against all odds manage to blow up the hotel then guy most likely would have been killed, but you can't know that, and you must, you're a hitman.
Dressing up as a chef? Yeah, sure, noone in the staff would notice that someone who's never worked there before suddenly turns up as the chef. In that case you would need to apply for a job weeks or days in advance, and the scenario never stated that you had weeks or several to finish the job. I think it's pretty much in the cards that this was classified information and couldn't have been availible weeks or many days in advance. And even if it was you'd still have no guarantee you got the job, and even if you got the job as even a chef, how would you know what food to poison? Not to mention eye witnesses. And you would have to have the skills of a chef, or you would get fired pretty fast. In that case it's much better to be the one who delivers the food to the rooms, however you can't know if they will actually order room service, so that's a bad idea. Besides you don't know who will consume the goods, maybe it's the guards, which would screw up that plan.
Oh and your package, what kind of explosives is in that can make it blow up an entire hotel? Is it perhaps a hydrogen bomb?
Oh and that air vent plan, you would need the plans for the entire hotel's air ventilation system and it would have to be big enough for you to actually pass through, which you can't know in advance. Besides even if you are lucky it's hard to pull of, so when you are in the vent in the hotel room, you have to get down to the hotel room, how? And if you manage that chances are the guard will hear sounds and you will get shot and killed.
Now try to come up with a plausible plan. You get this mission at 0800, and the politician will arrive some unknown time during the next day.