Nestlé for ethical reasons.
I would boycott The Sun but it's utter shite so they've never had my custom anyway. Does that count?! Same goes for Starbucks, McDonald's and KFC. Don't like meat or coffee so I suppose that has something to do with it.
All other actual boycotts are due to bad personal experience:
Debenhams due to an appalling customer service experience many years ago where I left sobbing and received a generic "tough shit" letter in response to my complaint.
Sky due to their utterly awful customer service and general incompetence.
Schuh because they tried to tell me that the heel falling off my day-old shoe was 'general wear and tear' and that's what they supply a spare set of heels for. I wanted to punch the bitch who tried to tell me I'd tampered with the shoes to make it look worse
Santander because they mistook me for someone with the same name (different salutation, different home address, different date of birth) and refused to remove her bad credit mark from my credit record for THREE YEARS. Wouldn't deal with me by phone because I "wasn't the account holder" NO BECAUSE I AM NOT THE FUCKING PERSON YOU SAY I AM and wouldn't reply to my letters of correspondence. Letter, after letter, after letter, after letter... and only managed to resolve their almighty fuckup when I got the Financial Ombudsman Service AND my MP involved. Useless pricks.
My mum hasn't stepped foot in a Mothercare since the staff refused to let her toddler (my DB) use the loo. She was heavily pregnant at the time, flounced off and hasn't been back since. That was 30-odd years ago.