Best Buying Source For Coin Purchases?

Dealers (LCS) and collectors know what they have and you will rarely scoop up something at a bargain basement price.
The US Mint is a good source if you want particular coins NOW. Sometimes their limited editions increase drastically in price, while in other cases you can get some items from a few years ago for less than issue price from other sources.
ebay is good, as many sellers list there and you'll find items that are otherwise elusive. If you're patient and cherry pick, you can make great deals.
Yard sales and estate sales are all over the place. Sometimes bargain, oftentimes overpriced.
Other online auctions can be good but be aware of buyer's premium and shipping charges.

I feel like I used to get some good deals off ebay in auctions but now everything I'm interested in is BIN with ridiculous prices. HA and GC have been my main source lately and I feel like I got a good deal on "some" of them.

You are missing coin shows from your choices.

I suppose that can be included under Dealers, but having so many in close proximity helps with both availability and prices.

Depends on what you're looking for really. I get my high-end stuff like pre-1933 gold from dealers, though ebay auctions are nice for finding cheaper stuff.

It depends A LOT on what you are buying.
If it is junk silver, large online retailers are probably the best. My LCS charges outrageous premiums and ebay is typically a bit overpriced.
Auction houses such as GC, HA, and Stacks are good for American classics and higher value world coins. LCS can be a good source for these coins (USA) as well, e.g. Gerry Fortin, etc.
For lower value world coins I like MA Shops, Delcampe, etc.
Coin Shows and LCS are obviously the best place to observe coins in hand. Prices can be negotiated, especially if you buy several coins from one dealer.

I'll admit to being a little confused with the OP's question, but I did vote, as the way polls work on this forum, you can't vote if you post a comment first.

If by "best" and "bang for your buck" you mean cheapest, or furthest below market value, then dealers are definitely not the way to go. They need to make profit, and they won't make much profit (and won't stay in business very long) if they keep selling you coins too cheaply.

If instead you mean "high quality coins that you actually are seeking for your collection", then dealers would have to be the top of the list. "In-person auctions" wasn't on the list so I'm going to classify that with "dealers" too.

Preferring the second interpretation, I voted "dealers".

I buy a lot of items from coin club auctions, too, which I guess would fall under "Collectors" in the polls, but these are rarely "core" items that I really needed in my collection.

However, most of my recent purchases have been from a "None of the above" source. I am a volunteer for a charity that occasionally receives donations of coin collections. It's my job as "the coin guy" to try to sell these coins to raise as much money as possible for the charity. Rather than just dump them all off at a local dealer, most often I end up buying about a third of the world coins myself, for my own collection. The agreement I have with the charity is when I buy coins from them, the price I pay is in between what a dealer would have given for the coins, and what I would have paid if I'd bought the coins straight from a dealer (so, in between wholesale and retail). It's win-win; I get nice coins cheap, the charity gets more money than they would have gotten from the dealers.