what chain should be used in HTB-Queue Tree system?
chain= prerouting...action=mark connection
or
chain= forward...action=mark connection
?
here are some links that i have already gone through but still confused:
http://wiki.mikrotik.com/wiki/Manual:HTB

i have seen in youtube videos some people use prerouting when marking connections and some use forward, so which one should i go with?
Regards
Azfar Hameed Khan
MTCNA, MTCTCE, MTCWE
and here is the packet flow diagram:

Mangle prerouting will apply to connections that will be forwarded and to connections for the router itself, while mangle forward won't apply to connections to the router itself.

I have a question.

has anyone tried Loadbalancing with 2 WANs and HTB-Queue Tree in the same router?
if yes? how did it go?
does it work?
I never tried though but according to me it will be very hard to do that because in HTB we force the packets to use the interfaces that we specify whereas in Loadbalancing the packets are sent to interfaces which are not busy.

waiting for your kind replies. Friends.
Best Regards,
Azfar Hameed khan

I've never "tried" this, but I can say that managing downstream bandwidth with a queue/queue tree is going to be very difficult to do well.
Upstream queueing for QoS purposes should be doable, because each WAN interface should have its own dedicated queue tree, and it will be easy to set the correct bandwidth on each tree, but downstream is going to be strange.

I would think that you would need to make a queue tree for downstream which has a parent of the HTB, with no bandwidth limit or else some number that's greater than the sum of both Internet links. Then you'll need a child queue for each ISP link you have. Then you'll need priority/non-priority child queues for each "isp queue" - it is the ISP queues that will need the correct bandwidth set for each provider's actual value.

When you set up packet marking, you'll need to do packet marks such as voip1, voip2, besteffort1, besteffort2, etc. And make the voip priority queue that is parented to isp1 queue match the voip1 packet marks.

As long as the two downstream amounts don't total more than the bandwidth of your actual LAN interface, then you should have no troubles.

Thank you very muck ZEROBYTE. 
Sir i have tried the load balancing and Queue tree on the same router and got the Upstream working, but couldn't control the down stream. i had to use chain forward and had to define out interfaces while marking connections and packets. And i had to be very careful with the order of the mangle rules as i had to stop the packet from being passed through to the next rule. anyhow i was able to control the upload speeds only. and one more thing i used ECMP load balancing and haven't tried any other kind of LB. but i think the results would be the same. i am planing to try packet marking voip and bestefforts and if it worked i will post the results here. 
here are some tips for those who wanna try LB & Queue tree on the same router:
1) using chain "forward"....gives you more options like specifying "in and out interfaces".
2) since the src. address is same, so be more specific with out-interface when marking connections and packets.
3) since there are 2 ISP's, so we will have to make double queues, double marking of connections & double marking of packets.
4) be very carefull with the order of firewall rules, otherwise Queue-tree won't work. stop the packet from being passed to next
    rule by unchecking the "passthrough" box when marking packets. 
happy experimenting Guys!!
Regards,
Azfar Hameed Khan