Telecom Serbia does not offer ADSL service to end users yet. You'll have to decide from resellers.
Regarding ADSL providers - resellers, opinions are somehow different, and there are no rules. It depends for example in which part of town your flat is (Telecom local headquarters for fixed telephony), and what your fixed line condition is, etc..
If you want a stable connection, I would reccomend YUBC.net. Price is higher then the others, but I'm satisfied with the quality of service and support. You'll have to pay initial setup fee 3000 Dinars, and fee for your service plan every month 5 days before the end of a current billing cycle.
The second reccomendation from me would be SezamPro and then Eunet (most expensive as always).
Wait for others to give their feedback too. In our language there's a lot of quality discussions and user feedback's here in forum, but I guess that don't mean much to you.
There are wars that can't be won.
IT'S ALL THE SAME,
ONLY THE NAMES WILL CHANGE.