Are you looking for the cheapest or the worthiest altcoin to hold? The cheapest one doesn't mean that the altcoin will be worthy. There are many cheap altcoins, but you must not only buy depending on the cheap price, but the worthy altcoin that has high trading volume, a good prospect for the future, have a real product, a team that always supports and manage the coin, and more others. Moreover, a coin that is listed on the big exchanges.
Well, if you are looking for cheap altcoins that have a good prospect for holding, I will prefer to:
1. OGN
2. Zil
3. DEP
4. CTSI