This is a good and specific question!
I have answered this questions in parts all over the place, however, this question is specific enough to warrant it's own answer which will be very direct and helpful to others.
First things first. Search is NOT about keywords. Google does not make direct term matches. Not even close. So please stop thinking in terms of keywords and start thinking in terms of whole language with value. In fact, I found that I had to retrain myself to use complete sentences with solid semantic value and less in terms of short staccato statements.
When thinking in terms of semantics, it is important to understand that the HTML of your web page will be parsed into DOM elements likely using a parser that uses XML or some other similar mechanism. When this is done, all HTML elements, any content within a single tag, will be given an id that identifies; the order of the HTML element, any parent child relationship of the HTML element to another such as with ordered and unordered lists, and any relationship of one tag to another such as header versus paragraphs. Keep in mind that the ids allow analysis with dependencies so that HTML elements that should not stand alone will not.
You list three examples, each slightly better than the previous.
To begin, I realize these are just quick and dirty examples off of the top of your head, however, it is what I have to work with so I will use them as is.
Each of your list elements caries little semantic value. When looking at the content of any HTML element, content within a tag, ask yourself, What about...?
In your case, you have A/B Testing, SEO, and Landing Page each of which say almost nothing. In regard to parent child relationships, these are all peer relationships and therefore do to add value to each other. If any of the list elements were nested, then parent - child relationships would come into play and relate one to another. Each of your list elements will be taken separately. In otherwords, these stand alone and say nothing. However, if you were to ask, for example, What about A/B Testing?, you would easily come up with a list item with greater semantic value. For example, How A/B Testing can improve CTR and Bounce Rates within search.
Semantic value comes in when you offer a subject, predicate, and object just like you learned in your English classes. It does not have to be verbose, just enough. For example, A/B Testing improves CTR and Bounce Rate. In this case, Testing is the subject, A/B is a modifier of Testing, improves is the predicate, Rate is the object with CTR and Bounce as modifiers. Semantic analysis of this example is actually more complicated than this, however, you get the point well enough.
This is a list within a single HTML element. In this case, semantics can apply long standing analysis of lists traditionally found in sentences, however, this list is ambiguous. Again, ask, What about...? What about A/B Testing, SEO, and Landing Page? This is incomplete from a semantics point of view.
This is far more complete and offers superior semantic understanding.
One warning however, if you are writing a sentence, pay particular attention to all of the punctuation. In this case, your example does not have a period. This, along with the new trend toward not using sentence case, and abbreviations can cause semantic analysis to be in error. While semantics can compensate for much of this, there is a real danger in not having your content indexed properly if your sentences do not provide all of the clues traditionally given.
We can use A/B Testing for SEO friendly landing pages., can be better written as
Use A/B Testing to improve SEO friendly landing pages. There is little value to we can and signals author involvement versus reader involvement. Search can prefer author or reader involvement depending upon the search. If you are instructing others, then reader involvement is far more powerful since search queries seeking information signal authority which involves content detached from the person (author in this case) and signal fact based content versus experience based content. Replacing for, a predicate, with a verb, improve as an action, better ties A/B Testing to the object pages. Keep in mind that SEO friendly landing is a modifier of pages. It tells us what kind of pages we are talking about.
To answer your question, your last example is best for search.
Do keep in mind that there are times where ordered or unordered lists can be necessary. As well, comma separated lists can also be necessary. You should not avoid these, just create them with as much semantic value as you can.
For what it is worth, you can train yourself to write content with better semantic value using Tropes found here: http://www.semantic-knowledge.com/tropes.htm It is free. While it does not offer full semantic analysis, it provides much of what you need to train yourself toward content that performs better in search.