Choose for extended tours mainly. Standard tours are quite boring after you did one. Mostly it's not much more then run through the distillery with a big group of people and hear the standard jibberjabber you hear everywhere.
I've visited 39 distilleries so far and my best experiences in the Speyside where:
Glenlivet. It has a great extended tour which I did in 2014. It's called the Definitive spirit tour nowadays I believe. Here you will be able to draw a sample straight from the cask (at least back then it was part of it). Sign up for the guardians of the Glenlivet and you will be able to enjoy a complimentary dram in the Guardians Tasting Room after your tour.
Furthermore Glenfarclas: It's pleasant to feel a real family atmosphere in quite a big distillery. Standard tour is good, extended is better. Afterwards there plenty to taste.
Benromach is also fun to see because of the small size. The only distillery where, Mashing, Fermentation and Distilling take place in 1 space.
Edradour is way too touristic imho. Blair Athol is also worth a visit. It's quite a picturesque distillery. Photogenic if you'd like.
Glendronach is not worth your time. It's very, very simple and also too commercial. They make great whisky but the mood onsite is soulless and they don't have a clue about how to do tours.
Also Aberlour and Cardhu are not that interesting to visit. Cardhu is very commercially driven, Aberlour just doesn't nail it with their tour.
Balvenie can be very interesting for you if you'd like to see a working malt floor for instance. Glenfiddich is next to it, you could make a combination out of it.
At Glengoyne I would Highly recommend the Malt masters tour. I know it's a little expensive but worth every penny. If you have the chance, ask for Arthur to guide you around. He is great. Firstly you will have an extended (private mostly) tour, and after that you'll be getting a master blending class in the tastingroom where you will create your own Glengoyne expression out of 5 different straight from the cask samples.First you will taste every sample a part before start blending. Excellent experience.
Strathisla is perfect for a warehouse tasting.
Also a great tip is to ask for a quick peek in the stillhouse when you are at a distillery. That way you don't have to do full tours everywhere but you still can see where the magic happens and ask your most burning questions. It works great and most of the time it's no problem for them and also free of charge.
And now for the best part:
You really should do the Dufftown walking tour with Michelle!! This is a guided walk through all the Dufftown distillery. Michelle has a backpack with her so at every distillery you will get 1 or two whiskies from that distillery. http://www.speysidetours.co.uk/dufftown-distilleries-walk.php You don't want to miss this. It also gives you de opportunity to see inside closed distilleries since Michelle has been permitted acces to several distilleries that are normally closed for public. I did the Connoisseurs tour and I don't regret it. Hell I'd even do it again when I'm return to Dufftown.
And last but not least:
Go to the Speyside Cooperage. This is a really fun experience in between all those whisky tours you will be doing. It is interesting to see how they make and refurbish casks. You can see every step of the process and ask questions to the guide that sticks with you. Those coopers work like beasts overthere. https://www.speysidecooperage.co.uk/