6 spells per level is just not going to be enough for vancian casting wizards. They could considerably cut down the size of the spell list by combining alot of redundant spells, though (i.e. mage hand could include open/close, silent/major/minor image could just be phantasmal image, charm person/monster could just be charm, etc).
If I had to choose six of the most iconic spells of each level, these are probably the one's I'd choose. Of course, alot of these spells could very well change level, be combined with other redundant spells, and so on, and I'm sure alot of the more situational spells (i.e. secret page, permanency, contingency, etc.) could work fine as rituals.
0th: arcane mark, detect magic, ghost sound, light, mage hand, prestidigitation
1st: burning hands, charm person, feather fall, grease, magic missile, silent image
2nd: invisibility, knock, levitate, melf's acid arrow, mirror image, web
3rd: dispel magic, fireball, fly, haste, lightning bolt, slow
4th: animate dead, arcane eye, charm monster, dimension door, polymorph, wall of fire/ice
5th: cloudkill, cone of cold, dominate person, telekinesis, teleport, wall of force
6th: chain lightning, disintegrate, flesh to stone, planar binding, true seeing, wall of iron
7th: delayed blast fireball, finger of death, greater scrying, mordenkainen's sword, reverse gravity, spell turning
8th: clone, horrid wilting, iron body, maze, Otto's irresistible dance, polymorph any object,
9th: bigby's crushing hand, gate, meteor swarm, mordenkainen's disjunction, time stop, wish
and of course one cannot forget the various summon monster spells.