In the past I’ve played exclusivly as Guile online and when I play against any serious competition. For a while now I’ve been thinking it’d be a good idea to have a second character to counter (or at least have an even match) against Guile’s tougher matchups, the big 3 being Blanka, Vega, and Dhalsim. What character will do the best agains these three characters? I’ve always had an intrest in playing as Ken, and it seems he’s a good choice for dealing with these three.
So play as Ken?
Gief gief gief
Chun or Honda.
^ SJV is awesome and/so these are good suggestions.
In vanilla ST, I think Boxer wouldn’t have been a bad choice either. Not sure how differently his matches stack up in SFHD though.
Something else you might want to consider is just fooling around with Blanka, Vega, and Dhalsim themselves. (Maybe you already have, in which case never mind!) I guarantee you’ll play against them muuuch better once you understand what they’re up to.
Lousy suggestion: Just play all of the characters, they’re all so much fun. You will have a great time but never win any of your games ever… like me! 
im trying to learn Sagat in addition to maining Guile.
The hardest thing is to not switch back to Guile when Im getting my ass beat using other char’s.
Sagat is a good choice. Chun Li has no weak matches as far as I know.
What I basically recommend is that you play your Guile as usual, and when you run into some inexperienced player online; instead of destroying him over and over for no gain, just switch to your alt and get some practice and give the other guy some a more competitive match at the same time.