I'm younger than most of the crowd here, and being 15, I won't be able to vote for another few years, though I figured I'd pitch in my 2p here in any case.
I support her because of –
Experience (re. Sanders supporters: there's a difference between length of tenure and experience gained as a result),
Her role in normalizing U.S. foreign policy as Secretary of State (the gist of Sanders' foreign policy feels like it's from 2008 – Iraq was a mistake, Middle East is a quagmire which we should leave alone, the Iranian deal is terrible because the Iranian government is terrible, delusion of "endless war"),
Her ability to effect change (Sanders, by contrast, literally did not sponsor a single successful bill during his tenure in the House – and he sponsored hundreds; his tenure in the Senate has been similarly unsuccessful).
Ability to recognize political feasability. Some have taken to pointing out how Clinton seemingly backpedaled on single-payer in the past two decades; what's at issue, though, is really whether she believed it could have been passed, not whether she believes in it. The fact is that the political environment is different today than in the 1990s, and there's little chance that something like Hillarycare would make it past a Republican-controlled Congress.
Electability. A lot of Sanders supporters point out how general election polls indicate that he'd trump Trump in the general if he came out victorious compared to Clinton. There's two failures in this assumption, though: a. that Republican leaders would allow Trump to secure the nomination, and b. that general election polls far from the election have predictive value far from it. I've watched the debates; Sanders's inability to not pivot to his stump speech is problematic. Clinton has gone easy on Sanders; I doubt he would be able to hold under national scrutiny, especially in a way that HRC has.
Political maneuverability. She was a relatively successful Secretary of State; she can't tout any specific major accomplishments, but having played a role within the system for years, she's familiar with the "ins-and-outs" in a way I don't feel with Sanders. The fact that Sanders remains such an ineffective legislator after decades in Washington makes me skeptical that he'd be able to accomplish much as president. Which brings me to...
Foreign policy. I've already noted that I'm critical of Sanders' overly simplistic foreign policy; there's little substance to it, probably because it isn't his pet issue. HRC is not a "hawk"; her approach as Secretary of State was pragmatic and versatile. She is undoubtedly one of the most knowledgeable candidates one could hope for on terms of foreign policy – and foreign, not domestic policy, is the area where the president has more direct control over; they can set the tone of relations with other nations, but cannot necessarily expect Congress to reciprocate in such a way.
I've got more to write, but I'm going to take a shower and I'll be afk for an hour or so after that.