Perhaps Dutch. It is a lot closer to English than German and the Dutch have a long connection with the English (being across the channel and a political and economical partner for most of the two countries’ histories).
Just because it is “the easiest” doesn’t make it easy though.
I found Indonesian a very easy language to learn. Now that I am learning Spanish and German though I may have found them even easier to start with.
Sometimes “ease” comes from accessibility. Having the language closely resemble your own doesn’t mean it will be smooth sailing. I have found German very easy as a language but it has been very hard for me to find a language partner or an “affordable/cheap” tutor (to make up for me not having a language partner).
Spanish on the other hand has been much “easier” than German because I can easily find either language partners or affordable tutors. Also finding language learning resources for Spanish is very easy as well.
Indonesian was “easy” for me because there are a lot of cognates with English, it is spoken how it is written and Indonesians in general are happy to coax you to communicate. All you need to do is walk a few hundred meters down the road and you will find a temporary language partner who will be happy to sit with you for hours and drink warm tea with you while trying to understand you and be understood.
Another way to judge “ease” coming from an English speaking background is by looking at the American FSI’s (Foreign Service Institute) categorisation of languages according to the instruction time they estimate it will take to obtain professional working proficiency (I am guessing this is equal to B2 level in the CEFR).
According to their guide it will take an average of 24–30 weeks to learn any Germanic or Romance language (Category I). However, it will take 88 weeks to get to the same level of proficiency in Chinese, Arabic, Korean or Japanese (Category IV).
I’ve known several native English speakers however that have either learned Arabic or Chinese (Mandarin) and have reached an impressive degree of fluency. Perhaps it did take them longer than if they were to learn a Romance or Germanic language. But then again, they were excited about learning Chinese or Arabic weren’t they.
So, in conclusion, the easiest language for you to learn will be the one you are excited about, can easily commit to learning and are ready to be patient with.
Learning any other language, especially if it is your first foreign language, can get really boring and joyless very quickly if you haven’t got a lasting motivation to succeed.
P.S. My tip to anybody asking this question would be to pick one, any one that is low in difficulty on the FSI list and you have easy and affordable access to. Then put your money on the table and sit down with a tutor at least three times a week.
Make sure the tutor can teach you in a communicative fashion, focus on communication, not grammar drills.
Stick to this program for 4–8 months.
Even if you decide that you won’t continue with that particular language, your ability as a language learner and your confidence to tackle the next language you choose will be a world apart from what it was.