Buy what nobody recommends but make sure it's not total junk.
Best coin to buy probably isn't on polo though because coins listed there are pumped already. Polo-coins are actually higher risk than those not listed there after all these months of pumping.
But that's the thing, they're all junk. It doesn't mean it's impossible to profit from any of them, but you have to get in and out quickly. Frankly I wouldn't touch any of them right now.
Most coins in Poloniex are junk and overpriced. I don't mind buying junk coins but not at these overvalued price. XVC crashed on Thursday after a long period of slow downtrend.
I am holding my bitcoin till most coins are below 10k satoshi again. Then maybe it will be time to accumulate some of those cheap altcoins.