just purchased mikrotik RB2011UAS-2HnD-IN . but have no idea what is the best adsl modem required to use this router with .. i am a newbie to mikrotik pls help.
the best ADSL modem is the one your ISP supports, better ask them.
We have been using Draytek Vigor 120 ADSL Modems successfully with our Mikrotik routers until Mikrotik make one or build it in 
It deos not matter what modem you have. Even if you do not know what modem it is (black box), as long as you know all L3 information (IP settings, PPPoE, DHCP etc…).
For the rest, you do not care about it. As long a s there is a UTP port coming out of the modem. The modem can be cable, xDSL, fibre, WiFi modem, LAN switch, or whatever exotic connection you have. It is really transparent.