Hi there,
I put this together because it's difficult to know which mods to buy and very expensive to try them all (trust me, i learned the hard way

)
This is for newbies really but if you're not new to x-cart and you haven't got these mods then your store is working against you and you should consider upgrading now.
Firstly I'll deal with
Admin Back-end stuff.
Admin/Customer Login
Not the most obvious of must-haves but a very handy feature if you do a lot of talking on the phone with customers and need to login quick and sell stuff to them:
This mod places a login button beside your customers name in your admin user listings. Clicking the link will allow you to login as that customer and browse your store as if you were them. A link is placed in the top right corner of your store allowing you to easily log back into your administration panel.
Marketing Manager Pro 3
The MOST IMPORTANT mod of any you can buy - all the tools you need to maximise your modified x-cart store and convert live visitors and new customers into cold hard cash - simply essential.
High initial outlay but if I can sum it up in words that describe how important it is, imagine trying to run an internet store without a web connection - it's simply that important an add-on to have.
Selling & Conversion Tools:
WebsiteCM Customer Conversion Package
A cocktail of the best upselling and conversion tools available. If you want to add value to your brand and increase the profits in your store then this mod will take care of it for you.
Consists of EZ Upsell, EZ Recommends & EZ Checkout, this package takes a sale and builds in extra products and options for your customers to buy relevant items which compliment their purchase and ensures that when they begin checkout, they're more focussed on seeing it through - VITAL.
Advanced Mini-Cart
I can't believe this mod was needed in the first place as an add-on but it is, so buy it and give your customers a more relaxing journey through your store. Advanced Mini-Cart displays better info than the standard mini-cart and allows for removal of products which is important in creating the feeling that your customer is in control of their purchases - so often overlooked because of fear of losing the sale but it actually works in your favour - empower your buyer and you increase sales.
Visibility & SEO
CDSEO Mod - Custom Dynamic Search Engine Optimised Links for X-Cart
Of all the SEO mods out there, this one makes the most valuable difference in terms of achieving real worl improvements in your search engine rankings through making your links not only appear as html links but making the URLs for products and categories more relevant to your store keywords ensuring the search engines reward you for supplying content correctly. It works, others do too, just not as well. Simply brilliant.
Google Base - Use it - it offers an extra 20% of visitors from Google and that can't be a bad thing for a free shopping engine can it! - the tools within MMPro 3 give you the best froogle (google base) feeds.
Now there are loads of other mods which I and many other developers and store owners use and have used and there are more which I view as important so if I get chance I will update this post but these mods will give your store the flying start it deserves and ensure that every visitor is maximised and that your store gains ground on the search engines quickly in order to help you get going.
If you have a mod in mind that you want advice or opinions on please reply to this post and I will do my best to assist.