Like almost any other question in trading you can ask around the theme of what's the "most profitable", the answer is that it depends on your skill-set. For me, daytrading penny stocks would be a recipe for the directions to the poor-house, because I'm not an expert on it - and that's putting it mildly. For me, even daytrading spot forex (not exactly a widespread success formula!) would be a more profitable prospect. For you - I don't know - and be aware that neither does anyone else who replies: it's going to depend on many variables, about none of which you've (yet?) given any information at all.
I think penny stock trading is less competitive than other instruments, because is so volatile, low liquidity and full of scams(most stocks in the OTC arena). I believe best traders, pros, firms, algos are in high priced stocks, forex, futures etc. I think for small accounts will be interesting to start in penny arena. IMHO
There is an old saying in poker 'If you can't spot the sucker at the table - your it". Penny stocks trade on an unregulated market, they do not report financials (pink sheets) - a lot of the companies are shells, in other words scams. You have to assume that someone can slam the price around wildly at will. You could get caught in a trading halt where your shares are frozen, the company could suddenly delist leaving you holding the bag. You are better off at your local horse races.
Penny stocks are a fool's game. Plain and simple. Don't let the sucker devil pitchmen convert you. It moves and trades based on bullshit, basically. A Joker's playground. There are other ways in the market to make a relatively steady and predictable income from trading...that is way more 'real'. But to answer your question...generally, loosely speaking...I would say yes...penny stock day trading is probably one of the most profitable things in the market. It all falls ...back ...on that: Risk vs Reward spectrum.