Ignite : Always a good extra damage output.

Ghost : Good for escapes / catching up.

Exhaust : Kinda useless later in the game with this type of build.

Flash : Really awesome to get close (surprise!) and shoot of hextech gunblade.

Teleport : Is fine when playing 5v5 (never uses it myself) but having combat / escape spells . gives you the edge in team fights etc.

I max mark of the assassin first (except shadow dance) since its a good harasser and farmer but i always have all my spells (cresent slash for farming and twilight shroud for escape / stealth)
after that i up each every lvl.