Really depends on how much you want to save.
If you're looking to put in $5500 or less (as an individual), then an IRA makes the most sense - very easy to set-up, not too many gotcha's.
If you want to put in no more than $5500 + 25% (20% for sole-proprieters), then a regular IRA and a SEP-IRA make sense.
If you want to put in more than that, then a traditional IRA + Individual 401K gives you the most tax-advantaged space (unless you make so much that your SEP-IRA contribution could be 52K). Here, where a lot people are striving for 75%+ savings rates on fairly high incomes, this is often is the best choice.
Then of course, you can add a taxable account to whatever plan you decide on, and may have to if you want to save more than you legally can put into tax-sheltered vehicles. Taxable accounts aren't too bad of a deal either, particularly when you get to the withdrawal stage if you're still living a frugal lifestyle.