Out of the three options listed above, there is non that is actually bad. If you make an investment at the ICO stage, you will get some bonus as an investor, hold to it and wait for its rise, sell and make your profit, or buy from exchange site and hold through the deep, or you also wait for the deep and buy more of the coin. Buying at deep time is better if you are not an investor into the coin/token during ICO stage.

What happens is that many people do not want to go through the trouble of investigating the coins themselves, it is quite ironic they want to obtain huge profits but they do not want to make any effort to get them, and that is why you see many people in the forum being scammed by so many icos, and this in return makes them even more desperate to obtain bigger profits in the future.

three answers in your three questions.

first -in ICO stage- yes it will be a good time to buy their coins because the price value of the ICO token is in a very low value and another good thing is they will give you a bonus of their token if you will invest to the ICO but the risky of buying altcoin in the ICO stage is you have not sure if that token will make a good shot if its already listed on the exchanger so make a double decision by picking a good iCO.

second -in the exchanger- you can also use the exchanger to buy altcoin by simply reading the graph movement of the coins that you want to buy if you know how to read candle stick that is better because you will notice if the coin that you will going to buy will be high or going down its price, by reading the movement of the coin in the graph before buying an altcoin gives you a profit as long as you have a knowledge doing this.

third -wait for the dip- this is another good choice to buy altcoin if you are going a long term hodl. buying altcoin in a dip price gives you a higher profit if you will hold it for a long time til its price goes to the moon.

all your questions in when is the best time to buy altcoin is all good just make your own decision weather what method you are going to do.

Cases are different, it all depends on the specific situation on the crypto market and the project itself. I've seen cases when it was more profitable to buy coins on a crypto exchange, the price was even lower than on the ICO. This situation was during the last strong decline in the crypto market.

i think the most profitable to invest in is the ICO stage , in the iCO stage yoi can buy their coin and it may double or tripple yoir income , it depends on the ICO but it is quite risky to invest .

every time is the right time to buy altcoin, because every year the price of altcoin always increases.

there is no way to find the best time to buy, invest or trade. it all depends on the circumstances and conditions regardless of the accidents that have occurred in recent months.
so, it involves experience and of course knowledge.

For me, the best time to buy any altcoin is when the value is dip because in this time the price of the prospected coin is cheap and once it recovers it will give you an assurance to generated huge profit. But during ICO stage, also, it is an ideal time too because many ICO's now has a big potential in the market cap. Although invest money during ICO's stage is risky in the future once their coin listed in the exchange and market it will give you an assurance to gain huge profit from your investment.

My experience is to buy altcoins when the bitcoin rate is low when investors' cash flow will be on altcoins instead of Bitcoin. And experience is that you should purchase altcoin in the top 20 according to Coinmarketcap to avoid the risk. Or you can buy altcoins since ICO but need to learn to avoid risk because now there are many ICO scam campaigns.

Any investment also has risky, and of course ICO is no exception. Choosing the time to buy altcoin is dependent on the preferences and abilities of each person. I think you should buy altcoin from the ICO period because then it prices will be more reasonable and there will be big promotions for investors. If you miss the ICO, you can still buy the altocin on the exchange but it is possible that the price will be different. You also need to choose the altcoin that has the potential to grow to get more profit