Just exclude the IP address from your drop rule. For instance if you want to allow 192.168.1.105 and drop everything else, you would have the following in the IP address field:
192.168.1.1-192.168.1.104,192.168.1.106-192.168.1.254
Or you could make a separate rule with the action set to allow and place it ahead of your drop rule
/ip firewall filter add comment="allow super youtube user" disabled=no chain=forward layer7-protocol="YouTube" src-address=192.168.1.105/32 action=accept
The most efficient (and the easiest to edit later) is to add IP addresses to an address list and exclude them from your rule. For instance, create an address list called YouTube and add 192.168.1.105 to it. Then in your drop rule go to the advanced tab, select your 'YouTube' list for source address list, then check the '!' box to the left of the list.
/ip firewall address-list add list="YouTube" address=192.168.1.105/32
/ip firewall filter add comment="drop youtube" disabled=no chain=forward layer7-protocol="YouTube" src-address-list=!"YouTube" action=drop
Don't forget your reciprocating rules. I hope this helps
Thomas