Light Red & Light Blue StatesThese states usually go to one party or the other, but only by a margin of 5-10%, so there's an outside chance of them flipping. Since Gary Johnson is polling much higher than Jill Stein, the light red states are the most likely to flip, but there are no doubt some Bernie supporters whose knowledge of Johnson ends at "he smokes weed," so there's at least some chance of him spoiling a light blue state for Clinton.
Light Red States: Louisiana, Georgia, Arkansas, Arizona, North Carolina
Light Blue States: Pennsylvania, Wisconsin, Minnesota, Oregon
Real Swing States
West Virginia, New Hampshire New Mexico, and Ohio are evenly split over the last 10 elections, with each party winning the state 5 times.
Iowa and Michigan have gone to the Democrats in 6 of the past 10 elections.
Nevada and Florida have gone to the Republicans in 6 of the last 10 elections.
Missouri, Virginia, and Colorado have gone to the GOP in 7 or more of the past 10 elections, but the margin of victory is usually small enough that it would take a much smaller shift in voter patterns for the Democrats to win than most other states would require.
If you live in a state that has a real chance of flipping and honestly believe that one of the two major parties is the lesser evil, by all means vote for the lesser evil. If you live in one of the states that's almost guaranteed to go to one party or the other, voting for the minority party is wasting your vote in my opinion, but it's your vote so do what you want with it. If you think there's a lesser evil but it's still more evil than you want to vote for, or if you'd prefer your vote to go toward helping a third party meet the finance threshold, you might consider looking into something like www.votepact.org, where you and someone who's reluctantly voting for the other lesser evil each agree to not reluctantly vote for the lesser evil. Since both evils "lose" one of the votes they think they're entitled to, your refusal to vote for the lesser evil doesn't help the greater evil.