why should I mine digitalcoin and not Litecoin or dogecoin?
It's a matter of preference. Personally i think litecoin is a little slow, just bitcoin by 1/4th? not really interesting imo, it's only where it is due to being early in on the scene. Doge's price gets thrashed around and is a plaything for whales, not really useful for anything other than speculative daytrading.
I like DGC because it's always been treated like an open-source project. Its ecosystem isn't completely wrecked and the original developer did a great job ensuring network integrity, it's also starting to attract other developers who're interested in keeping it viable long term. The way the economy currently is the value is set to rise in proportion to the amount of community interest vested in it. Compare these two pie graphs:
https://chainz.cryptoid.info/vrc/#!rich
https://chainz.cryptoid.info/dgc/#!rich
You'll see DGC is far more distributed than VRC or other PoS (piece of shit) coins. It's at a level where it can be taken seriously, an ideal digital coin don't you think? :p it should also be mentioned that there was no premine or instamine, the bagholders behind DGC had to either mine or buy them on market.
For the record the alts I'm comfortable holding long term are BTC, PPC, DGC, and MYR. There some other notable alts (and a lot of crap ones) but these four have the most potential for success imo.