If you just want to keep investing on the cryptos every month without cashing out in the near future then it will be the bitcoin for your preference or you can invest on the altcoins and do short term trading with it to make your profits in small intervals but it needs your effort also.
Honestly, for long term hold, bitcoin seems to be the thing. Sure, a lot of people may say there is a need to diversify, but as things are going right now, I still find it pretty hard to find altcoins to be diversifying into until I basically start seeing something substantial in them when it comes to the usage of their product if they have one at all.
Certainly, at this stage, the market is highly speculative and there is no doubt about that, but when it comes to having your mind at rest, even if you want to invest in altcoins, you should be putting more percentage into bitcoin and lesser percentage into altcoins you have basically studied to know there is a chance for possible future.