The "best" way? Depends. On the style, genre, type of guitar. Sometimes a chord sequencer is the way to go - it's convenient, fast and requires almost no skill. MIDI chord generators are another option. Or you can play the chords on a keyboard, which takes a lot of practice. Or you can plunk in MIDI notes in the PRV and use a guitar library with pre-sampled strums.
That's because this is a two-part question: first, how to create a convincing strum, and second, how to string strums together in a convincing strum sequence.
First of all, a big +1 for SM4. Includes both 6- and 12-string guitars and ten body IRs and an easy-to-use sequencer. You can create up to 12 strum patterns and switch between them with keyswitches. A useful instrument and a real bargain at $46 (!).
That said, SM4 is really best for tracks where you just want an acoustic guitar to fill in the cracks, rather than something up-front, e.g. the only instrument track in a singer-songwriter type composition.
Most of my sampled rhythm guitar tracks are played on a keyboard. Once you get the hang of that, you can use any sampled guitar instrument.
Another good one, but considerably more expensive ($179), is Orange Tree Samples' Evolution Steel Strings
. All OTS guitars have a strum mode and a flexible strum sequencer.
All of the above is based on the assumption that you're talking about acoustic
guitar rhythms. For electric guitar, there are GOBS of options. Check out Indiginus' Renegade
, a favorite of mine for both rhythm and lead. And of course, OTS has a bunch of suitable offerings as well. I'm especially enjoying OTS' Rickenbacker 12-string
If you're specifically into heavy genres like one of the many flavors of metal, then that opens up a whole 'nother set of possibilities. However, those genres don't lend themselves to sequenced chording like acoustics do. For chugging power chords, I tend to hand-plant those in the PRV. In those genres, you might as well go for a product that's specifically targeted at that market, such as Shreddage
from Impact Soundworks.