I'd like to disagree with the notion that HSiFS is one of the easiest games. It's only easy if you know how to abuse releases, which most new players won't. In terms of danmaku, it's actually very difficult from stage 4 and onwards, with Okina in particular having some of the trickiest spellcards in the series (ESPECIALLY the Spring Final). I mean, it's still a very good game and isn't "unforgiving" like UFO or LoLK, so I wouldn't fault anyone for starting there (in fact it's my favourite Touhou game lol), but it's definitely not one of the easiest for beginners. WBaWC tho? Yeah, that one's pathetically easy.
Personally in terms of starting games, I'd recommend MoF. This is for a couple reasons. The first is that it's the first game in the "modern" era of Touhou, where ZUN really started to refine the gameplay. Not to mention that the story of the previous games is really self contained and you don't really need to play them to understand what's going on. The other reason is that it's a very forgiving game, but not to the point where it throws resources at you like candy like in WBaWC, or has pathetic excuses for danmaku like in TD. Tying bombs to power is a good way to get a new player used to the fact that bombs need to be used at pivotal moments and not just willy nilly, as it will penalise them for bombspamming. At the same time, however, they get a lot more bombs that they can use than in the other games, and even a free bomb at 5.00 power that doesn't drop their power at all. This is good for getting them to actually use the bombs in the first place and not just hold onto them forever thinking that it's a good idea to save them for later (a "later" which might not even come). A lot of people complain about the bomb system in MoF and SA, but that's mainly because they're just used to the other bombs and don't like dropping power to use them. However, if a new player starts with those bombs and moves over to the more traditional bombs later, it transitions much more easily.
Plus MoF has a killer soundtrack even for a Touhou game, lol.