I personally love long lets play if the person is entertaining or at least talks, I hate when people don't talk I get bored and quickly change. Game reviews are a great idea as well, people are always looking for information on new games so I would say go for this for sure if you can do newer games. Montages are something I would kind of stay away from until you get a bit of a following, it feels like something more of a side project unless you are REALLY good at it and can create killer ones right off the bat.

If you think you're funny / entertaining, I think you should do long let's play. Game reviews is a really good idea, especially if you can make the review entertaining and informative. I don't recommend montage video for starters, because doing a montage video is a little hard, except if you think that you're already good at editing.

You could both long let's plays and reviews. For Example, do a let's play series, and then end it with a summary/review of the game.
It could work well, as people visiting your channel might see the review and decide to watch the let's play, or choose to avoid it because it's not what they're looking for.

You could try both let's plays and reviews and decide which one you like best. If you're fun and entertaining, viewers will stick around and want more, no matter which type of videos you make.

Some, like "punch lines in a nutshell" which is basically a montage with the 'best of' in one episode. Some, like watching the whole thing.

Personally, I would enjoy any kind of video, as long as you can keep it interesting and fun. What's more important, make what you enjoy doing the most. Find your own unique style and give it your best! Good luck!