I've tried most of the highly regarded creams on the market except 1 (ironically, the one that is reputed to be the most slick: Santa Maria Novella V3). In my experience creams are harder to compare than soaps because the lather quality is more individual from person to person (depending on their technique), but here's my top 5 list of worthy mentions in relative order of most slick to least slick:
1. (reserved for the very slickest I come across - probably will end up with SMN V3)
2. XPEC: The slickest cream I've yet tried. This is the one I'd recommend to straight razor shavers and lovers of slickness, largely because of these qualities and it's ability to produce a great-looking post-shave complexion. The new version is a bit more tricky to lather, but it still performs very well.
3. Tabula Rasa: (This is described as a soap but I consider it a cream due to its airy consistency). Very slick and extremely protective. The scents are seriously great and avant-garde, and are obviously created using quality fragrance/essential oils. Worth checking out IMO, especially for folks with sensitive skin due to the Stimu-Tex ingredient.
4. Castle Forbes: My favorite is the lavender. Not only is the cream calming to sensitive skin (lavender oil is awesome in this regard), but the lather is exceptionally dense and slick mainly because it can take a lot of water. Out of all the UK creams in the typical style (users of multiple UK creams will know what I mean by that), this is the one I'd go with since its much more dense and protective than Trumper's, T&H, St. James, and so forth.
5. Acqua di Parma Collezione Barbiere: While this is one of my favorite creams on the market, it isn't the slickest, but certainly has enough slickness to do the job in complete comfort. This seems to operate on the principle of beard-softening. Once you lather this soap and take a blade to your stubble, you can easily tell that your whiskers are noticeably softer and easier to cut. Post-shave is always impeccable (probably the best I've tried in creams), and in-shave performance is protective and enjoyable. The scent is love it or hate it, but I actually prefer it to the old Colonia scent (blasphemy, I know). Great stuff!
These are all great products, and most of them will outclass a lot of the other available alternatives. If you love top performing creams, you should definitely make an effort to try each of these. YMMV, IMHO, etc.
