I hold a significant amount of Ark and Edgeless, I think they both have the potential to reach $30-40+ million market caps within a few months.
Ark seems to have a really strong team of 30+ and are pretty ambitious: they are creating a whole ecosystem with bridged blockchains (SmartBridge), usability in mind, decentralized storage and databases (IPFS and IPDB), and more. It recently had a big jump so it might be wise to buy once it has stabilized.
Edgeless is about online gambling, which is a quickly growing industry. Plus, online gambling and crypto go well together, due to crypto's better anonymity (depending on coin), availability to anyone with internet, and fast, cheap, secure transactions. Now Edgeless seeks to make that even better by being decentralized on Ethereum and having a 0% edge (0.83% in practice, for blackjack at least). And holding Edgeless tokens is encouraged: they act as lottery tickets in monthly drawings for 40% of the Edgeless profits.
For other, probably riskier choices with bigger upside, you can look into Incent, Blocknet, Shift, and Darcrus. All have $2.5 million market cap or less.
Incent appears to have hit a bottom and has a low market cap of $1.8 million (I think probably just 1.5-2x its ICO price, if I'm not wrong) and are trying to create a decentralized loyalty/rewards system. I have not looked much into it but it is certainly trying to solve a new problem.
Blocknet is focusing on being the internet of blockchains (like Ark's SmartBridge). It was almost killed because of the Poloniex delisting but it still has active development and 2.5+ years of maturity. It recently rose from a nearly-dead $200k market cap to $2.5 million market cap in the last few months.
Shift is a 1.5 year old Ethereum-turned-Lisk fork (possibly first for Lisk?) and are trying to implement decentralized websites through IPFS. They already have an alpha version working on their testnet and regularly integrate new Lisk features. I think Lisk was trying to do IPFS as well at some point, though I'm not sure how far they've gotten. This article mentions they might have it in a few months:
http://forklog.net/decentralized-startup-for-decentralized-apps-interview-with-ceo-of-lisk/). However, Shift is appealing because of its low market cap of $2 million, and Javascript/IPFS combo could be nice for web development. If they get the public IPFS out before Lisk, and it works well, it could quickly rise up to $4-5 million cap or more. It is still kind of a sleeper coin because of the small team and little marketing, but I expect it to grow a lot in the near future.
Darcrus is focusing on decentralized databases for businesses. It is less than 6 months old and has a low market cap of $1 million (about 2x ICO price). They have a WAVES token and the Darcrus dApps may also be run on the Stratis platform, which is hugely successful (now ~50x ICO price). I think it's just the work of one developer for now, but they seem fairly competent and have worked on Stratis, Supernet, and NXT projects before.