What do you mean by 'easiest'? I've only ever been to the Tour (twice, seven stages total), but I found it quite easy to follow if you planned well in advance. I've seen a flat stage, a time trial and several mountain stages, and by far the mountain stages were the best for live viewership. If that's not an option (you said you were leaving early on), it's mostly gonna be 200 guys gliding past so I'd try to see a sprint finish, which would probably be pretty exciting. Or that cat 2 climb near the end of stage 3 looks like it'd be alright, the place to see some attacks.
You should probably go to whatever's closest to where you'll be anyway. If you have a bike it makes getting around easier once the roads are closed to cars. Although Paris-Nice in March is probably not as busy as the Tour in July.