Without knowing what features you're looking for, it's difficult to recommend any specific knives regardless of price-point, but I'll tell you the features I look for and try to give an example or two that fit within those parameters.
The strongest locking mechanism in a folding knife is a lock-back. The manufacturer that offers the most choices in lock-backs with blade steel, shape, size, and grind combinations etc. that I'm aware of is Spyderco. Here's a
search on Blade HQ for all of Spyderco's lock-back models. I am not shilling for Blade HQ per se, but I have made several purchases from them and their pricing and customer service has always been great as far as I've been concerned. Any decent web-based knife shop has various search filters though, and you can research the different models on most of them.
Now, a lock-back knife
does have the most potential for strength, but it's not necessarily the "best" tactical/defensive knife, because the locking mechanism makes opening the blade a little stiffer, though it can still be easily accomplished with one hand as long as there's a thumb-stud or thumb-hole (like the Sypercos). But one-handed closing is a bit trickier. I gave up long ago trying to close my lock-backs one-handed. Not necessary to give a detailed explanation of the whys and wherefores here, but suffice it to say that cuts and blood were involved in the decision-making process. That's why I recommend a frame-lock for the best compromise between strength and ease of opening/closing one-handed. That opens up your choices to nearly every major brand, as well as to most blade steels, handle and scale materials, ambidextrous and tip-up/tip-down carry options etc. The most popular material for frames/handles in frame-lock designs is titanium, and that fact alone may limit your choices significantly because of your desire to stay around $140 bucks. Decently-made titanium frame-lock knives typically go for $180 bucks or so on up to whatever you're willing to spend. Put another $100 bucks on the table and you can buy a very high-quality production knife with titanium frame lock from Zero Tolerance, Benchmade, Spyderco, Reate, DPx, Kizer, Quartermaster, Lion Steel and several others, that also have premium blade steels such as S30V, VG10 (both of my personal favorites), ZDP189, M390 and, again, several other high-end steels. Here's
another search on frame-lock models filtered in price between $100 and $250. There are a few in there at or under your price-point, but research the steels used in the blades, the company's reputation, country of origin etc. before going with one of them. Most of the brands I listed above are going to be at least $50 to $100 bucks more than you'd like to spend, but really, if you buy one of those and it suits your purposes, the knife will very likely outlive you.
If $140 is the absolute max you can spend, filter your searches on "liner-lock" and look for some of the steels I mentioned above. Liner-locks are a good design, relatively strong lock-up, just not quite as good as frame-locks. To my way of thinking, blade steel is more important than any lock design anyway.
Some other considerations:
Do you care how much the knife weighs? (Materials matter if you do.)
How long do you want the blade to be?
What shape do you want the knife to be (spear-point, dagger, clip, drop-point etc.)?
Do you want an auto or assisted opening mechanism?
Do you have any scale materials that you particularly like or dislike?
Answering these (and other) questions before you actually go shopping can help you narrow down your searching.
Good luck. Hope that helps some.
Blues