Loyal, funny, and always there. Even with his hatred for Angel (which I don't think was completely based on jealousy, nor was it completely unreasonable), he would (usually) do the right thing in the end, and that would continue with Spike (in truth, I wonder if this isn't a flaw...it worked for the show, but wouldn't in real life), though I should give the disclaimer that a couple of things he did that are hated by many are things I support him in doing (or at least see as unintentional).
He was the everyman, but had his life on track far faster than most. For me this emphasizes his bravery and loyalty.
His insecurities are a double-edged sword. I think they motivate recklessness rather than courage at times, though there are times that it's courage, typically in defense of others. People forget how many times he has saved the lives of others and the world (not just against Dark Willow), at least played a crucial part in doing so.
This also fuels his jealousy which is a huge flaw, and can make him do bad things (or sometimes the right thing, but for the wrong reason, or at the wrong time). And, in my headcanon (though I know from a meta-story perspective it's because they wanted to make him the butt of jokes), he never develops his strengths, not even learning how to fight that well (beyond the reside of being a soldier while under a spell). I think there's enough there to assume he could've been a warlock even more powerful than Willow had he chosen to go down that path (but I'm glad he didn't, though I do have to count this as a flaw that grated on me at times).
People who say he is an evil "Nice Guy" don't know what a Nice Guy is who doesn't actually know the meaning of altruism and feels entitled to women as if they were objects and essentially treat as vending machines ("if I do X, then she is obligated to do Y, or she's taking advantage of me!"). Sometimes Xander is incredible with his self-control (especially for a high school boy), and if he felt entitled to female bodies then he'd have reacted very differently when that love spell went wrong (which he suffered a lot from, which I point out due to the baffling insistence he faced no negative consequences while also overlooking that he didn't take advantage of it--and his vengeance wish was a lot less destructive than Cordy's later on which was thankfully erased, in addition to him suffering the most from that magical vengeance) or with the Potentials (as he dreamed of them), among other things I could point out.
And going with the insecurities thing, people, including himself (and even myself, it's just occurring to me now), forget how often females hook up with him with surprising frequency, their often being supernatural in some way aside--and there'd been normal relationships had the supernatural not freaked other women away as happened more than once, or he wasn't already off the market (I'm thinking of that land lady who seemed to be brazenly inviting him to cheat on Anya with her that he ignored). Given how cute and having it together he is, I'm not surprised. At least as long as he stays off the booze that is a problem with his family (and the actor who plays him). Even when he and Willow had that dumb affair he was wracked with guilt over it.
This is a strength many lack, he sees people as people even when he desires them. And it's part of his loyalty and doing the right thing in the end though it may cost him dearly, and he knows that he'll be the unsung hero for his part (he actually seems to forget his own numerous life and world saving contributions himself). As he matures he accepts this with grace.
On a side note, mention the above I'm reminded of a guy who took film school classes told me his own interpretation on that: Angel and Spike (and, to a lesser extent, Riley) represent the action, danger, passion, and where all is for the taken. They're the fantasy (whether you prefer shining knights or "misunderstood" bad guys with the "hottie diff" as Kim Possible once put it that distinguishes them from garden variety jerks, and is mocked in that season 7 ep where Willow says she can see into that one guy's soul and Anya points out it wasn't his soul she was staring at), though the real world analogs of those people will work out very differently in real life to their sorrow.
In contrast, Xander represented the mundane future after they've turned off the TV (I'd add if they were lucky, the smarter girls would hone in on the Xanders fast before others woke up to the difference between fantasy and reality) that they didn't want to think about, the boring stuff (until you're old enough to appreciate those things). And from a meta-perspective, that's a strength of Xander as well (which isn't ruined because Xander isn't a saint, everyone is going to make mistakes they regret in addition to racking up other regrets), though it causes many to hate him for what the boring, mundane reality he represents (as well as perhaps of one's self, but Xander is better than many people out there).
This is why most romantic leads in fiction are going to be Angels and Spikes rather than Xanders. From a meta-story level that's a weakness on Xander's part, but seeing beyond that, I also consider it a strength.