My favorite card in my lineup is probably my BOA customized cash rewards bc of the 3% online shopping category.
I would shoot for Amex, but find a card that works for you.
I mean... what's your actual goal? Looking at the cards you have, you have a flat rate 1.5% with the Quicksilver, and the solid rotator with Discover It. The others are store cards.
Are you looking for cards that will help your get more back out of your normal spend patterns (such as cards for groceries, gas, dining, travel), or just cards to improve your credit for the sake of improving it with high spend limits, such as NFCU doles out?
Are you looking to cover your bases with "types" of cards (Visa, amex, MC, disco)... or? How do you feel about annual fees? Cashback vs points?
Biz - 
@BMW_Jess wrote:Hi! I have been lurking here for quite awhile. Learned a lot of tricks, and tips and have slowly been building my credit up. I have been thinking about adding one or two new cards to my portfolio. Suggestions on what to add?
Currently I have the following cards/limits;
CapOne QS1 card - $2,300
Discover It - $5,000
Victoria's Secret - $4,300
Lowe's - $35,000
CPM CC - $1,500
My scores on myFICO show 708, 709 and 703.
Welcome to My Fico Forums @BMW_Jess ... or at least out of the shadows! 
Your card lineup and scores are helpful, but tell us more about what you're hoping to achieve. Are you happy with the cards you have now? Have you identified some area of heavier regular spending where you'd benefit from having better rewards, such as a restaurant or gasoline card? Do you like rotating category cards? Do you want higher limit cards or are you happy with your current limits? Do you ever carry balances and want a low-APR card? In the past, have you had any bankruptcies or delinquencies involving any particular lenders? Where do you have current banking relationships? Do you prefer cash back only or would you like to branch out into some form of travel rewards? Are you averse to annual fees or would they be okay as long as you get enough overall value from the card? All of these things might help us give you better suggestions.
I do have one suggestion. While I've enjoyed my Capital One Quicksilver experience overall, I think most everyone (especially those not doing travel point systems) can benefit from a great basic 2% or higher cash-back card. The 1.5% cards are commonplace, but 2% is becoming more and more accessible. Over time, that extra 1/2 or more percentage can add up.
Here is a recent thread discussing some of the better 2% or better cash-back options:
https://ficoforums.myfico.com/t5/Credit-Cards/2-3-Cashback-CB-Credit-Card-CC-List/m-p/6197473
Here is a thread discussing the 5% rotating category cards:
We'd need more information for specific answers, mainly about what your spend is and/or if you have any specific goals such as "maximum credit limit" or "get in with big banks". That said, a couple of things to consider. 1) If you are under 5/24 (which refers to opening five cards in the past 24 months), I'd look at Chase's lineup to see if they have anything you think would be a fit. The reason I say this is that once you start building, you are likely to go over 5/24 and have to wait a long time for Chase. I have 800+ scores and can't get a Chase card due to this unusual rule. 2) If you are 1/12 or 0/12 (refers to cards opened in the past 12 months), you might want to look at US Bank for similar reasons. 3) Amex is a great company to be in with, IMO. Personally, I'd look at them next. They have many options for different types of uses, and can grow nicely. 4) If you haven't yet, you might consider trying to join Navy Federal (relatively easy) or USAA (more stringent). Both have some interesting card options and are relationships you'd want to build.
I would avoid store cards at this point. By which I mean a card that is only valid at that store. A cobrand card that is a Visa, MC, or uses the Amex network is fine if it fits your needs. Store cards are scored a bit differently, and you have plenty of them IMO.
Just some thoughts. Seems you're off to a good start. Good luck!
@unsungivy wrote:Muahaha, I beat you to the gist @Aim_High 😘
@unsungivy ... the new My Fico Credit Ninja! 
It seems that other members often beat me to the punch since I tend to type longer before posting! 
Welcome @BMW_Jess.
Could you give us the dates that your accounts were opened… either that or their ages in months or years/months?
Have you been using your QS1 regularly? If so, you might be able to upgrade it to a no-fee card.