What standard should I use for the naming of my MVC controller services?
I don't think it matters as long as you are consistent. I personally prefer:  _{servicename}Service format. ex. _referenceService

public class ContentsController : BaseController {
        private IContentService _contentService;
        private IPageService _pageService;
        private IReferenceService _referenceService;
        private ISequenceService _seqService;

        public ContentsController(
            IContentService contentService,
            IPageService pageService,
            IReferenceService referenceService,
            ISequenceService sequenceService) {
            this._contentService = contentService;
            this._pageService = pageService;
            this._referenceService = referenceService;
            this._seqService = sequenceService;
        }