Have been saving up AP what should I buy?

[Dutch Yourself]

I have not spent any of my points and I'm fine waiting even longer to spend if it's worth it. Currently at 70k AP. What do you recommend?

[Fel]

The very first thing most people end up picking is the improved loot filter because as you auto-kill more and more titans the basic loot filter is no longer able to filter what you don't want and still give you what you do want.

Other good first things are the red heart for extra exp and the yellow heart for extra AP, but usually they come after the loot filter (because you need to throw them in the daycare for a decent amount of time to get the real effect from those two hearts).

[Kinson]

I've been playing for about a month, so not as big of a vet as others here.
I agree though, Improved Loot Filter is the best choice. All the previous sets of armor you've gotten to level 100, you can filter out so they don't even go to your inventory any more (If you watch the Adventure screen as you kill, you'll see those items still drop, but they get filtered out before getting to inventory). As Fel pointed out, very nice for filtering out titan loot.

Alternative is to spend $$ on the Beginner's Package, which gives you the Improved Loot Filter in it, and gives you a lot of AP, which you can turn around and buy a heart. Some people won't spend any money on a free game, but for me, I ask myself when buying stuff from a free game, "Have I gotten $xx amount of enjoyment from the game?" (and take into account if you spent money on it beforehand, then you ask, have I gotten $xx amount of enjoyment from the game since the last time I bought something?)

[Nyxeth]

Echoing the above, grab the improved loot filter first.

If you're deadset on not spending any cash I'd suggest the Yellow Heart next and throwing it in Daycare (if you have it unlocked). Yellow Heart increases your AP gain and at level 100 you gain the AP bonus from it permanently; which means you'll get AP for other things quicker down the line.