There are many aspects, for example, services, their commissions for transactions and support of necessary tools for you, that should be taken into account for your investments to be profitable. Thus, your investment will be the most profitable.

To begin with, let's consider ways of buying cryptocurrency. You can buy through bank cards, through electronic payment systems (these include QIWI, WebMoney, YouMoney, Payeer, AdvCash, Epay, Neteller, Capitalist, Paxum, NixMoney, Skrill, PayPal, etc.), cash in various currencies or from your cell phone balance.

Centralized Cryptocurrency Exchanges (CEX)

👍 The advantage of CEX is a wide choice of cryptocurrencies and has the lowest inter-exchange fee.
👎 The disadvantage is custodial service, withdrawal of fiat currencies from 3%, as well as token withdrawal fee can be high.
Available options: a limited set of EPS.
Examples of CEX: Binance, Coinbase, FTX, Kraken, Bitfinex, Bitstamp, Poloniex, etc.

Decentralized Cryptocurrency Exchanges (DEX)

👍 Advantages: no storage required and full anonymization.
👎 Disadvantages: choice is limited; no ability to work with fiat currencies.
Available options: tokenized assets only, as well as stabelcoins.
Examples of DEX: Uniswap, PancakeSwap, dYdX, DODO, Balancer, SushiSwap, etc.

Peer-to-peer (P2P) sites

👍 Advantages: Lowest commission.
👎 Disadvantages: custodial service; limited options; multiple transactions required to purchase large volumes; risk of getting "dirty" money.
Available options: bank card processing; limited list of cryptocurrencies and stabelcoins.
Examples of P2P: Cryptolocator, Binance P2P, LocalBitcoins, LocalCryptos, Paxful, Monabey, Bitpapa, HodlHodl, etc.
With the help of aggregators, you can sort out the variety of offers on several sites at once. Monetory is similar.

Exchangers

👍 Advantages: a lot of exchange options, some of them do not require verification.
👎 Disadvantages: custodial service, average commissions.
Available options: a lot of methods of deposit and withdrawal, including cash
Examples of exchangers: Mine, NetEx24, WmExpress, 365Cash, ExHub, NixExchange, Kassa.cc, etc.
You should choose exchangers via aggregators such as BestChange service because of fraud risks.
Exchangers have a balance between security and profit, if you want to get more profit, you can take a risk. Exchangers have advantages of P2P in terms of less attention to AML / KYC procedures and low commissions.

Electronic Payment Systems

👍 Advantages: convenience, minimal transaction fees.
👎 Disadvantages: custodial service; average commissions; many EPS are licensed and may exchange information with tax authorities.
Available options: bank cards and other wire transfers.
Examples: PayPal, WebMoney, Advanced Cash, Payeer, Perfect Money, etc.

Telegram Bots

👍 Advantages: simple and easy to use
👎 Disadvantages: too many fraudsters, which can mimic real services; no control by aggregators; average commission.
Available options: bank cards, some EPS, a small selection of cryptocurrencies.
Examples: @Chatex_bot, @bitpapa_bot, @Prostocash_bot, etc.

Cryptocurrency wallets with inbuilt purchase

👍 Advantages: convenience, economical transactions.
👎 Disadvantages: high commission, sometimes there are custodial services
Available options: bank cards [Visa / MasterCard].
Examples: Trust Wallet, Atomic Wallet, Blockchain.com, Jaxx, Exodus, etc...

Brokers, banks and fintech

👍 Advantages: all transactions are legal.
👎 Disadvantages: custodial services, which are fully controlled by tax authorities; average fees; very small selection of cryptocurrencies.
Available options: bank cards and other non-cash transfers [Visa / MasterCard].
Examples: Revolut, Robinhood, Banxa, Nexo, etc.

ATMs, bitcoin terminals [crypto machines]

👍 Advantages: complete anonymity.
👎 Disadvantages: physical accessibility; high commissions; possibility to buy only bitcoin.
Available alternatives: in cash, sometimes you can withdraw to bank cards or EPS.
Examples: Coinstar, BBFPro, etc.
There is also an option to buy directly from people. For example, from friends or acquaintances, as well as through advertising sites. This method has a lot of risks and it is better not to use it. More and more often investors buy cryptocurrency from miners, but also this way is risky and with high commissions. Cryptocurrency from miners are "clean" and have a high price.
Each method has its advantages, disadvantages and of course risks.
In order to minimize the loss of funds it is necessary to carefully double-check the data before buying, you should not use little-known services and before buying to make a few test purchases.