I'll recommend you to stay at Sofitel Bangkok Sukhumvit for a all round experience. Good Service with a nice lounge, close to BTS too.
If you are on abit of a budget and want location and abit of service, would recommend Novotel Pleonchit as it will fit your criteria.
However if you want just location and budget, not much great service, I would say Novotel Siam Square as that is in the centre of Bangkok.