Focus abilities aren't a good way to judge which specialization to take. For the assassin, his mark ability can go toe to toe with any focus ability any day. With the tempest, the only reason his focus is so great is because of an exploit which hasn't been fixed yet. Both specs are very enjoyable.
For the archer, the most fun is probably the artifacer. Done right his abilities will all have nearly zero cooldown, and his stamina will always be a plenty. All his specialty abilities add utility which can make encounters much more interesting and enjoyable. It adds to the archer rather then changing its playstyle, but thats just my opinion. I love the tempest, but I thought it much more fun with daggers. I could spam throwing daggers 5 times with a fire flask if I ever wanted to fight at ranged. Now that was fun.