Ive got most the BK line. Ive read the following a couple times before and I agree that ones first becker should be what is the staple of the entire BK line and that is the BK2 period. Why? First nothing batons like it,second its a quarter inch thick of coated Ka Bar 1095 cro van which is now arguably better than ESEEs 1095 cro van which is saying a lot. Third, yes its heavy but its short length wise despite its weight and heft its fairly compact to carry at the same time. Fourth, edge retention on the BK2 is the best in the entire line simply cause the steel is so thick. Being so thick you have a ton of steel for a custom grind should you so desire and still have a ton of steel left to do even more. Fifth, when you buy your BK2 you also buy the option Ka Bar made upgrade scales at the same time. Ka Bars upgrade micarta scales are nothing short of phenominal. I have them on all of my BK's. They are the ultimate in grip and comfort you will find for any BK they feel soooo good sometimes when I am stressed Ill grab one of my Bks and just hold onto the handle for awhile AAAHH!
Sixth, the sheath the BK2 comes with may be a bit on the crappy side but it is still compact. all the long BKs have sheaths with a bit to much extra material going on for a knife that is already long and wide. There are more custom made BK2 sheaths out there than for any other model I highly recomend the recent BK2 sheath by 710.
Lastly as a self defence blade. I guess its that knives like the BK7 and 9 and even 4 are pretty big and intimidating that many make the mistake of not realizing that out of the box a BK2 is still near razor sharp. The length of the BK7 and 9 can very easily work more against than for you in a tactical situation. If you are forced to stab someone in the gut with a BK2 that will most likely go a lot deeper than you thought it would its going to make a MASSIVE wound channel with one blow much more than any other BK. If its a high stress self defense situation and all you have is your BK2 adrenalin is pumpin and you go for the gut hard that BK2 is going to penetrate all the way to the handle and when you pull that very sharp heavy quarter inch thick slab of 1095 cro van out of said attacker guts will spill out the wound will just be that massive. Even worse is if you penetrate fully then TWIST 90 degrees then pull it out you will essentialy gut them near totally. So there is where the BK2 makes up for its lack of reach vs the other BKs
Finally if you really wanted to make a point one could take every other BK model in the line and baton through each one of them with a BK2 and a mallet. I saw a YT video where some dude used his BK2 to baton through an ESEE 5 and it worked....scary stuff but that is the first BK you should get and why...good luck